US-China relation is a major topic in international politics. They are the rivals and compete each other in various fields. One of the most severe potential flashpoints is the South China Sea dispute, where China claims historic rights and building artificial islands and US opposes it. The milestone in the dispute is the July 2016 Arbitration. The core legal elements of the ruling were in line with US legal policy. At the same time, the arbitration rejected China’s legal claims. Since the Tribunal’s ruling lack an enforcement mechanism, it seems to be ineffective to settle the dispute. However, it effects the legitimacy-related aspect of the US-China relation. The study of US hegemonic legitimacy opens this aspect. US hegemony primarily derive from its material superiority at the end of the WWII. US hegemony was legitimated in certain social context. This US hegemonic legitimacy was constituted by the substantive base of shared values, US adherence to open and accessible decision-making process, its strategic restraint and the successful outcomes. US hegemonic legitimacy conserved and stabilized US hegemonic status. Therefore, material predominance and legitimacy were the two pillars to US hegemony. The July 2016 Arbitration legitimated US legal claims and behavior in South China Sea while rejecting China’s legal claims. The Arbitration effects on the legitimacy-aspect of US-China relation, not the material aspect.