Today's strategic structure of Northeast Asia was formed in the early 1950s, in which the U.S., Japan, and South Korea make up of the "South Triangle" and China, Russia (Soviet Union), and North Korea make up the "North Triangle." These two "Triangles" have been the main focus of confrontations and conflicts during the past fifty years. However, the Six Party Talks on North Korea's nuclear issue shows cracks in the "South Triangle." South Korea finds more and more common view with China, its former enemy state; on the contrary, although the U.S. has always been considered as South Korea's firmest ally, South Korea finds it more and more difficult to continue to follow America's stands on this issue. Meanwhile, the relationship between South Korea and Japan continues to worsen because of the historical reasons, which even America, as the leader of the "South triangle" finds tough to deal with. In this paper, the author analyzes the backgrounds of changes in Northeast Asia's strategic structure and explains why South Korea shows so much "centrifugal" tendency in the "South Triangle" and why strategic cooperation between China and South Korea became possible from the perspective of geopolitics, political economics and interior politics. The paper also discusses the influence of China-ROK strategic cooperation on forming a new strategic structure of Northeast Asia.