1. Introduction Choice of law is one of the three pillars underpinning private international law. It refers to the determination of a governing law in a disputed international case. The issue of contractual matters is a remarkable one in most jurisdictions, given the rapid development of international business in the trend of globalization and the consequently increasing international contracts. Unlike a domestic contract, an international contract is not governed by a uniform set of substantive rules. Except for some cases (e.g. international sale of goods), international contracts are normally governed by the freedom of the parties and by the conflict of laws solution given by private international law. At present, however, there is no uniform set of choice of law rules for international contracts at a global level, let alone the unification of law rules for international contracts. Consequently, the solution for disputes arising out of international contracts has to resort choice of law in the countries having jurisdiction, and the relevant national legislations in various jurisdictions are the main legal sources of choice of law in the contractual matters. In recent years, the legislation on the issue of choice of law regarding contractual liabilities has advanced in some jurisdictions. It is noticeable that the European Union and the People's Republic of China (mainland), the two remarkable roles in the international trade market, both have made great progress in legislation concerning this subject. The European Union promulgated a new regulation, the Rome I Regulation, to govern the choice of law issue arising out of contracts. This regulation is the successor to the Rome Convention on the Choice of Law for Contracts, the equivalent instrument applied in the European Union countries at present. This legislation is an action to replace the original international convention with a Community law. It is notable largely because the European Union is a union rather than a country. As one of the world's greatest economies, both the internal trade and the external trade of the European Union have a reference to this new regulation. As the Rome Convention, the Rome I Regulation is also an effort to the unification of law rules on choice of law as to international contracts. Although the basic structure is retained and the changes are not obvious from a general look, the application of the new regulation not before long will bring a brand-new era of choice of law in international contracts throughout the European Union. On the other side, the Supreme People's Court of China promulgated a judiciary interpretation, which is The Rules on Related Issues Concerning the Application of Law in Hearing Foreign-Related Contractual Dispute Cases in Civil and Commercial Matters. This instrument is applied on the basis of existing fundamental frame of choice of law rules in the Chinese legal system. The starting point of the Rules is the judicial proceedings of the disputes of international contracts litigated in Chinese Courts. Therefore, an in-depth study of these two important regulations is considerably useful, especially upon their value in legal practice. The Rome I Regulation is the authority followed by the courts in the Member States of the European Union in order to identify the applicable law for international contracts, while the Rules plays the corresponding part leading the Chinese courts. Such practical significance is not only for persons living within the two jurisdictions, but also for persons from other countries who may be entangled in international contract cases heard by the courts in Europe and mainland China. The central theme of this article is the choice of law systems on contractual matters in the two jurisdictions. The discussion mainly focuses upon the Rome I Regulation and the Rules, from a perspective of the development in Legislation. The problem of determining the applicable law of international contracts is very complicated, owing to the diversity of connecting factors, the intended participation of parties with uncertainty, and various contractual issues that can arise. However, all the problems ultimately direct to the ascertaining of a law, which is an alternative option, either to apply a choice made by the parties, or to be decided in accordance with a connecting factor by the courts. These two methods turn into the core sets of rules supporting the system of choice of law established in various jurisdictions, one is the parties' freedom to choose the applicable law, the other is the rules taking effect in the absence of the parties' choice of law. They are the key provisions in the Rome I Regulation as well (also in the Rome Convention), which are presented as Article 3 ("Freedom of choice") and Article 4 ("Applicable law in the absence of choice"). Similarly, they are also reflected in the crucial provisions of the Rules, i.e. in Article 3, Article 4 (freedom of choice) and Article 5(applicable law in the absence of choice). This article only concentrates on these provisions, and others are left out of the scope of the study. Furthermore, the study focuses upon the primary rules provided under the two methods, keeping other indirect issues away from the study work. This article attempts to make an analysis on these provisions from the perspective of legal certainty and flexibility, which are two main goals pursued by laws. The balance between legal certainty and flexibility exists everywhere, and the choice of law system is not immune from this contradiction. Part II of this article would introduce the main theory of legal certainty and flexibility, and the relevant viewpoints in the area of choice of law. In Part III, the author discuss the regime of choice of law for contracts in the European Union. After the introduction of the historical background from the Rome Convention to the Rome I Regulation, the author focuses on the changes of the two key provisions mentioned above and discusses the provisions of freedom of choice and applicable law in the absence of choice respectively. The research on the two crucial sets of rules is carried out both in the theoretical level and in the practical level. The original provisions in the Rome Convention and their application in practice are expounded, on the basis of several reported cases involving the application of Article 3 and Article 4. The cases studied are from the courts in the United Kingdom, from the year of 1996 to 2008, considering the relevant decisions have increased since late 1990s.8 The arguments and opinions of the judges are also taken as instruments to illustrate the changes in the two articles in the Rome I Regulation. They are used to measure the purposes of the modification and the legislator's intentions, and consequently to postulate the imaginable effects brought by the alterations. The author attempts to further penetrate into the significance of the changes in the two crucial provisions, in the perspective of the equilibrium between legal certainty and Flexibility. After the discussion of the regime of choice of law in the European Union, Part IV turns to the regime in mainland China. The content starts with the development from the original general rules in the law framework and their operation in practice Then the author concentrates upon the system established by the three crucial provisions in the Rules — Article 3, Article 4 and Article 5, which provide the two mentioned methods. This part also contains the theoretical analyses of the provisions and the judicial practice in the Chinese court. The author has selected some typical cases involving choice of law in contracts from the courts in Beijing, Shanghai, Guangdong, Jiangsu and several other provinces, from the year of 2005 to 2008, both before and after the application of the Rules. The application of choice of law rules before the Rules and the status quo are reflected through the analyses upon the selected cases. The balance between the goals of certainty and flexibility is still the focus of the discussion. An in-depth comparative study on the two regimes in the choice of law systems of the European Union and mainland China is carried out in Part V. The research is mainly done on the basis of the certain provisions in the Rome I Regulation and the Rules, including the similarities and differences between the provisions and rules applied. The comparison also centers on the basic framework of the two methods with the holistic view and detailed view. The perspective of certainty and flexibility is remained in this part. At last, the conclusion of this article is stated in Part VI.