Based on the features of network economy and the theory and practice of antimonopoly law, the author proposes the impacts of static characteristics, dynamic characteristics of network economy and trade form in network space on the definition of relevant marketFurthermore,the author reveals that the market share is not a key factor to define dominant position in network economy The key factors are network economic effects, standard and intellectual property of the key facilityFinally,the suggestions for legislation are given by analyzing relevant articles of Anti monopoly Law of People’s Republic of China