This study focuses on the network externality effect related to the platform supplementary services. This study designs the network externality of platform and suggests a supplementary service adoption model. Additionally, this study examines the moderating effect of demand forecasting for the platform. Using AMOS program, a structural equation modeling has been used to analyze the research model. The findings can be summarized as follows : First, we find out the structural relationship among the factors (usefulness, perceived value, purchase intention) affecting adoption of the supplementary services. Second, positive perception of platform flow can promote the platform interaction. Third, positive perception of present users based on platform can arouse friendly evaluation in the platform interaction. Fourth, loyalty to the platform brand can improve the perceived usefulness of supplementary services, but cannot lessen the resistance to supplementary service cost. In addition, the moderating effects of demand forecasting for the platform in the path leading from platform factors to supplementary service factors were identified. In conclusion, traditional brand strategy may be effective in platform marketing activities but the extent of performance in the strategy can appear to be quite different. Therefore, taking the relationship with network externality into consideration should be involved in the marketing strategy in platform.