It has been largely acknowledged that preliminary ruling procedure, as a jurisdictional mechanism of cooperation between the Court of Justice of the European Union and National courts, which is prescribed in Article 267 of the TFEU as amended by the Treaty of Lisbon, dispels contradictions between the EU law and the domestic law of Member States and ensures a uniform application of EU law among all Member States. Therefore, this mechanism has become one of the most important procedures in the European Union Law. This thesis discusses the preliminary ruling mechanism, which includes its background, significance, functions, historical review of the related constitutional rules, the specific procedures from the angle of the CJEU and from the angle of national courts respectively, and the legal effect of preliminary rulings. Then by a comparative analysis, the current thesis reveals the nature of the relationship between the CJEU and national courts under this mechanism is independent but binding. Such nature of the relationship reflects the intergovernmental and supranational characteristics of the EU. What's more, under the preliminary ruling mechanism, this cooperation relationship deepens the supranational characteristics of the EU it reflects, further promoting the EU's integration objectively.