This article analyzes how China has used the concept of 'core (national) interest' in diplomacy, particularly in dealing with the South China Sea and the Senkaku problems. China began to use the notion of 'core interests' to appeal the seriousness of the Taiwan problem to the United States in the early 2000s and expanded the application of the term to Tibet and Xingjiang in the latter half of the first decade. It was included in the U.S.-China Joint Communique of November 2009. In spring 2010, New York Times' report of Chinese high officials characterizing the South China Sea as a 'core interest' attracted broad attention as a manifestation of Chinese assertive foreign behavior. However, as the assertiveness was criticized as counterproductive within China, the term came to be defined in a limited manner, but its application to the South China Sea was not explicitly denied. Since spring 2012, there were several indications that the Senkaku islands belonged to the 'core interests' but their official nature was unclear. On these issues, Chinese expressions came infinitely close to using the term to emphasize seriousness of the issues while remaining unclear about their official nature to retain diplomatic flexibility.