Regional economic integration and foreign direct investment (FDI) have become the most striking focuses today. This study investigates FDI agglomeration under regional economic integration by game theory analysis and literature analysis. Among many mechanisms of regional economic integration, the market size effect is one of the most important ways to affect FDI. As FDI varies with the types of regional economic integra-tion, it is difficult to determine the effects of regional economic integration on FDI in general.