Defining relevant markets is the foundation of establishing main antimonopoly regimes and the key issue in enforcing antimonopoly law, which often reflects the leniency or strictness of enforcement. In the process of defining relevant product market, the main factors to be considered include physical function and use purpose of product, product price, consumers’ preference and substitutable possibility of product supply. In defining relevant geographic market, the main consideration involves transportation cost and product characteristics, product price, consumers’ preference and barriers to market access. On the occasion of forthcoming enforcement of the Antimonopoly Law of China, the enforcement authorities should draw up a specific rule of the definition of relevant markets.