Facial expression is regulated by complex neuromuscular system with mutual link among facial muscles under control of facial nerves. In clinical diagnosis of facial nerve palsy, simple and quantitative assessment of facial motion is strongly required. In this study, a diagnosis system for the quantitative assessment of facial nerve palsy has been proposed using the optical flow method that provides the degree of precise movements on the whole facial area. Right-left ratio of the facial motion for various expressions obtained from the optical flow analysis is very effective for diagnosis of the asymmetry of facial nerve palsy in clinical usage.