Power grid interconnection has gained attention in Northeast Asia (NEA) as a means to effectively utilize the abundant renewable resources in Mongolia. This paper quantifies the potential economic and environmental benefits of deploying massive wind turbines and solar PV in Mongolia for power exports. The author uses an NEA-wide multi-region power system model formulated as a linear programming problem. The analysis considers power systems characteristics, such as the seasonal and daily electric load curves of the NEA regions. The simulation results show that the large-scale Mongolian renewables contribute to significant CO2 reductions in NEA. China, in particular, benefits from a significant reduction in coal-fired generation. However, huge investments would be required for the massive renewables and cross-boundary transmission facilities, pushing up electricity supply cost. The relevant planning organizations need to carefully consider these environmental opportunities and economic barriers before implementation. This paper also investigates the economic impacts of transmission route circuity due to avoiding transmission through the Democratic People's Republic of Korea (DPRK). Our results imply modest effects of the circuity on the total system cost; availability of routes through the DPRK would not significantly increase the benefits to the NEA system of integrating massive renewables in Mongolia.