The present study was conducted to ascertain characteristics of mandibular movements in patients with SQM, observe the kinetics of masticatory muscles using a four-dimensional (4D) muscle model, and kinetically investigate the etiology of Square Mandible (SQM). As results, 1, In the maximum opening position, location of the condyle was beyond the articular tubercle for volunteer, but within the mandibular fossa for SQM patient. 2, While the temporal muscle of volunteer was markedly expanded, that of SQM patient was not. 3, In both volunteer and SQM during left lateral excursion, the right mandibular condyle moved to a position slightly before the lowest point of the articular tubercle. The 4D muscle model showed that the cause of limited mouth opening in SQM patient was insufficient expansion of the temporal muscle, and not dysfunction of the opening muscles. Insufficient expansion of the temporal muscle stresses the masseter muscle and leads to hypertrophy of the masseter muscle and hyperplasia of the mandibular angle, resulting in the unique facial configuration.