This thesis is aimed to study the relations between China and Vietnam before and after the 1979 “Punitive war”, the war which occurred from February 17 to March 16, 1979. The main objective of the thesis is to find out factors which lead to such war and to explain China’s behavior in continuing relations with Vietnam, its geographical and historical closet neighboring country. Hereby, the theory of Realism and the concept of Sphere of Influence are used as a framework of the thesis. The study has found that the factor that forced China to use force against Vietnam from February 17 to March 16, 1979 was its strong concern to maintain the security in its own sphere of influence. Due to Vietnam aggressive and dangerous behavior at that time which affected China’s interest in this region, China decided to use force in order to punish Vietnam. During the “Punitive war”, China did not cut off its diplomatic relations with Vietnam. The diplomatic relations between the two states remained but at the lowest level. After the end of the war, both China and Vietnam tried to negotiate with each other under the mistrustful and conflicting attitude to create good relations. Anyway, both countries tried continually to eliminate the obstacles. Finally, on November 10, 1991, China and Vietnam agreed to improve their diplomatic relations to normal level and, thereby, the good relations between these 2 countries restarted and last until now.