In these days energy demand is dramatically increasing on the developed and developing countries. Even though the electric power efficiency is gradually improved, the limited energy resources and the shortage of land to construct generations have been existed for long time ago and the environmental limitation like the Kyoto-Protocol is originated. To overcome these obstacles, there is one solution through the cooperation on the national-wide. That is, we should meet the energy problem with having strong relationship among nations. The technological and systemic cooperation on electric power system is much more important than before. It is sure that there are economical and systemic advantages over the limitations when interregional power systems are operated well. Simply, power system interconnection gives the eco-friendly energy' applications and the flexible operating power system. In this paper, the first is focused on the algorithms on the power system and the other is based on the economic side. It is mainly to discuss the tertiary control illustrated over each first and secondary controls existing on two control areas.