Most methods of assemblability evaluation are based on the experiences in shop floors. They do not provide generic rationale of the evaluation results. We believe assemblability of mechanical parts depends on their behavior while they are being mated. This paper proposes an assemblability evaluation method that analyzes the state of parts in the mating process based on the assembly model. The state of parts is represented by a set of contacts which one part has with other parts. Mating process can be described by the series of these contact states. In each contact state, parts in contact form a mechanism. By simulating possible behavior of the mechanism the transition from the contact states to the other contact states is analyzed. The result will be a good basis for assemblability evaluation. A prototype system is in development using solid modeling system and kinematics analysis software.