The author has studied the duration of silent period of the masseter muscle using electromyography in order to obtain the information that is necessary to diagnose the patients with TMJ dysfunction syndrome objectively. The 55 normal subjects and the 40 patients with TMJ dysfunction syndrome were selected for this study. The results were as follows : Duration of silent period of the masseter muscles in normal subjects were 24.4 msec in males, 24.0 msec in females, and mean of $24.2pm3.19$ msec. Duration of silent periods of the masseter muscles in patients with TMJ dysfunction syndrome were 39.4 msec in males, 41.3 msec in females, and mean of $40.5pm7.01$ msec. There was no significant difference of the duration of silent periods between in the right and the left side of the masseter muscles, and between in the males and the females (p>0.05). Duration of silent periods of the masseter muscles in patients with TMJ dysfunction syndrome were significantly more prolonged than those in normal subjects (p<0.01). Borderline of silent periods of the masseter muscles between in normal and patients with TMJ dysfunction syndrome was 31.5 msec.