A prototype system of facial expression synthesis has been developed by using facial muscle modeling method. The system can synthesize dynamic process of facial expressions and display it as computer graphics. When synthesizing facial expressions by muscle modeling method, the parameters such as the muscle contraction rates and time variations should be decided beforehand. In this study, these parameters were decided by two subject experiments. The first experiment was conducted to decide muscle contraction rate, and the second one was to decide time variations of the muscle contractions. By using these parameters, it was confirmed that the prototype system could dynamically synthesize facial expressions and display them in real time.