This paper，after tracing in brief the origin of China-Japan dispute over some part of the East China Sea and the history of joint development there，discusses the pattern of setting up a joint development zone as well as the international practice in this kind of development，and then points out that the position on the part of Japan regarding the joint development in the disputed area is neither in line with international legal principles nor in accordance with the general practice of the international community. Although China and Japan have reached common understanding with regard to the principles guiding the joint development in the East China Sea，out of political desires of both sides，this author yet deems that the delimitation of the joint development zone is still the key issue governing the process of the joint development in the East China Sea，which can not really become ‘a sea of peace，cooperation and friendliness’ unless Japan relinquishes its position on the ‘central line’.