It is certain that improvement of the relations between South Korea and North Korea through the mutual efforts of only two Koreas is limitative. It is undeniable that the U.S. policy towards the Korean peninsula plays a determinant role in the formation of the trilateral relations among south Korea, north Korea and the U.S. The two Koreas' mutual efforts for improvements of South-North Korea relations should be implemented more strategically than before. If South Koreans are fully to understand the structural environment of the trilateral relations, the mutual cooperation between Seoul and Washington is compatible with independent efforts for improvements of the relations between the two Koreas.