This article examines legal questions of what will happen on the Korean peninsula if North Korea collapses without war. Since the end of cold war, the Democratic People's Republic of Korea (DPRK) appears to be on the verge of disintegration due in large part to dictatorship under Kim's family and disastrous economic conditions she faced. Some possible scenarios for collapse are advanced; from soft landing to hard landing. The soft landing scenarios result in gradual reunification in accordance with the Republic of Korea. The “hard landing” scenarios cause tremendous suffering, increased instability. On the other hand, in each case of the sudden collapse of North Korea, South Korea would face various legal issues in relation to its engagement to the North. The first is legal constraints on the use of force. If South Korea, or even international community, engages in North Korea during the sudden collapse, she must act based on international law concerning the use of force. Thus, legal justifications of the use of force, such as anticipatory self-defense, humanitarian intervention, and intervention by invitation, would work as constraint as well as justification. The second is the role of the R.O.K-U.S. mutual defense treaty. Since the treaty provides “armed attack” as a precondition to resort to the treaty, it is very hard for both countries to use their forces based on the treaty when the North collapse without any armed attack. Thus, both countries should take account this problem when they make the revision of the Mutual Defense treaty. Finally, if South Korea wants to transfer North's collapse to unification, she must consider international law seriously. North Korea's collapse would bring enormous strategic interests, objectives, and concerns of China, Russia, Japan, the U.S. and R.O.K. Thus, achieving unification during the sudden change of the North is not a easy task. However, because of the balance of power in North East Asia, international law would play very important role for South Korea to achieve its goal. This is the reason, South Korea should prepare emergency plan including legal aspect of the sudden collapse of the North. As professor Chayes once noticed, international law would play, during the time of trouble, law as constraints, law as justification, and law as organization.