Mainland China has been developing and maintaining its sea power after its economy is booming. Its maritime legal structure has been established, and maritime claims vis-a-vis sovereignty in the East China Sea and South China Sea are continuously made. After Senkaku Islands were bought by Japanese government in 2013, Mainland China implemented its Air Defense Identification Zone (ADIZ) in the East China Sea. However, the legal status of ADIZ affects the measures that a costal state could adopt against foreign aircrafts flying in that air space. This article explored the legal status of ADIZ from the perspectives of international convention laws and customary law to conclude the jurisdiction and sovereignty that a coastal state may claim in the ADIZ. Furthermore, this article discussed the backgrounds of why the East China Sea ADIZ was established and its impact on regional security.