The eastern enlargement of EU has attached a great deal of attention over the years, because the eastern enlargement of EU is one of the significant events related to the changes in political and economic pattern of world after the cold war. The eastern enlargement, as an important form of European integration, is still an ongoing project by EU for now. It provides not only the considerable opportunities but also the challenges to EU. The reasons leading to the eastern enlargement of EU are manifold. Nowadays, EU is the biggest regional organization with highest level of integration, to research for the rapid development of EU will have an important significance to regional integration theories. This thesis mainly aims at study why EU decided to carry out the eastern enlargement and what factors have the most impact for EU to carry out the eastern enlargement. The qualitative study method is adopted to analyze the questions. First of all, it reviews predecessors’ research related to the eastern enlargement to form a general impression. Secondly, it introduces the regional integration theories and puts forward research hypothesis. Thirdly, it describes the favorable conditions of international situation for EU’s eastern enlargement after the cold war. Fourthly, it examines the eastern enlargement of EU through the study and comparison of Turkey and Poland. Finally, it concludes the reasons for eastern enlargement from the perspective of whole EU. This thesis argues that the eastern enlargement of EU is a result of the combined effects related to political reasons, economic reasons, culture reasons, social reasons, and the role of Germany and France in promoting the eastern enlargement of EU. To a certain extent, the political reasons play a more important role to promote the eastern enlargement of EU, but the influences of culture factor in the eastern enlargement process can not be ignored. Keywords: EU; European Integration; Eastern Enlargement; Poland, Turkey, Regional Integration Theories