This study attempts to evaluate the possibility of power cooperation through the simulation analysis of electricity markets in Northeast Asian regions using the Generation-Transmission Maximization (GTMax) modeling. GTMax is a useful tool for analyzing power interconnection among countries and a software to maximize net revenues when an equilibrium for power trade and operation of power system is reached under the physical and institutional constraints. And it also contributed to building strategies based on hourly trade volumes, costs, and revenues by players participating in competitive electricity market. From the simulation results, this study indicates that the effect from interconnection with North Korea or Russia to domestic Korea electricity market is not so big. However, direct connection of transmission network from Korea to Russia has a clearly positive effect on power cooperation. Moreover, it is expected that power cooperation will be done in a way South Korea mainly imports. To maximize the effect of power transfer, it is absolutely necessary to expand the transfer capability of transmission line. Nonetheless, in CASE 5 (interconnecting South Korea with North Korea, Russia and China) scenario, positive effect in average LMP in South Korea is not likely to happen compared with CASE 2 (interconnecting South Korea with North Korea) scenario. And positive effect in average LMP in South Korea has been appeared in CASE 6 (interconnecting South Korea with North Korea, Russia, China and Direct transfer from Russia to Korea though power network) scenario.