This study presents the preliminary economic assessment for the power system interconnection in the Northeast Asian countries, The power exchange model considering is established. A mathematical model for calculating the net present value(NPV) and benefit/cost ratio of power system interconnection is developed, The model is also considered seasonal average load curves, plant capacity, voltage, line length of each country and the type of interstate electric ties. In the model net profits are composed of investment deferment or substitution benefit of power plant caused by the non-coincidence of seasonal and daily load curves, environmental benefit by the reduction of CO2 emission and benefit by the differences of electricity charges or power production costs. Major costs are summation of the construction costs of converter stations and transmission lines, and operating and maintenance costs.