The influence brought by the granularity of task decomposition on the key factors of enterprise aliance was studied based on system dynamics method. These factors includs inter-restriction, product manufacturing time, collaboration fee, Alliance leader benefit, capability and compatibility. Causality diagram and system dynamics model were established for these factors. This paper also developed a simulation program for the granularity of task decomposition based on the system dynamics model and validated the effectiveness of the proposed model by a qualitative analysis. The results show that it is a workable method to research the granularity of task decomposition in enterprise aliance using system dynamics theory.