The international system is a kind of informal social system that exists and affects its internal actors. Awareness of the international system is essential for each country to be able to adapt to surviving and developing. The article clarifies basic aspects of analyzing the international system and perceptions of the major theoretical schools about the international system including- Realism, Liberalism, and Constructivism. Based on that theoretical bases, the article clarifies the roles of the US and China in the contemporary international system from 1991 to 2022. The US from a single superpower that plays a leading role in creating the contemporary international system tends to decline relatively due to the rise of China. China has shifted from having to adapt to the international system to becoming a challenging actor and participating in the adjustment of principles of the contemporary international system.