Northeast Asian countries such as China, Japan, and Korea have tried to build their FTA and participate in different mega FTA such as RCEP and TPP. TPP became CPTPP as post TPP version after the US withdrawal in 2017. Compared with other regional economic blocs such as the EU and USMCA, their economic interests are rather divided deeply and related to political and security issues in the Northeast Asian context. Therefore, it is more difficult for Northeast Asian countries to build their own economic integration closely particularly compared with the EU. It must be wise to investigate the possible economic cooperation in the region that is one of economic hubs in the world. In order to meet the research targets, various methods are used such as a method of critical analysis of literature, an inference method, and a method of statistical analysis that include quantitative and qualitative analyses. This paper deals with possible FTA between Korea, China, and Japan that could be a starting point for the regional economic integration formally. It also argues what are three nations’ economic interests by participating in the mega FTAs such as RCEP and CPTPP as well as whether the two Mega FTAs can play roles in creating competitive or cooperative relations between three Northeast Asian economies or not. Finally, it analyzes the best scenario to participate in the multilateral FTA and mega FTAs for each nation.