Abstract: Under the theory of two-sided markets, online-payment platforms are subject to complex competition constraints. Due to reasons including positive cross-side network externalities, strong bilateral complementarity, and asymmetric pricing structures, a direct application of the conventional methodology for relevant market definition to online payment platforms shows its limitations and therefore needs to be adjusted and developed. Based on the distinct features of the online payment industry and the stage of development of the entire payment service industry, two options, short-term and long-term, can be taken as the methodologies for the definition of relevant product market. In the near future, it can be defined by the actual demand of the consumers for the functions and uses of payment service according to the transaction scenarios where online payment provides services. In the long term, when the online-payment industry has developed to a mature stage, the cluster market method, considering various payment services of the platform as the same relevant market in the form of clusters, can be adopted. Under the control of relevant policies, the geographical market should be restricted to the domestic market in general. The impact of technological innovation on the relevant temporal market should be given extra consideration.