On May 1^st 2004 the EU experienced an unprecedented and the biggest-ever expansion in its history. This enlargement had profound impacts both on the new CEE member states and on the EU itself. Generally speaking，EU accession contributes a lot to democratic consolidation of the new member states，with the exception of Hungary，which demonstrates the limitation of the EU’s political influence on the new member states. In addition，the EU accession promotes the economic development，enhances the economic potential and improves the international status of the new CEE members. However，the eastern enlargement has both a positive and negative impact on the EU itself. In the political and security fields，the eastern enlargement expands the community of peace，security and European values，but it at the same time leads to “enlargement fatigue”. In economic and social terms，the eastern enlargement has positive effects on EU economic growth，competitiveness and the free movement of labor，however，the problem of the division between East and West remains unrseolved. The enlargement as well helps the EU decision-making mechanism become more flexible and efficient，but the lack of transparency still exists. As to the external relations，the eastern enlargement extends the area that the EU’s external policy covers，but the diversified preferences and interests from different member states make coordination in decision-making more difficult.