I investigated the optimum ultrasonic therapy of the masseter muscle for temporomandibular disorders. Deep tenperatures of the bilateral masseter muscles and forehead were measured non-invasively at rest and after ultrasonic therapy. The right masseter muscle of each subject was exposed to a continuous sonic wave of 3 MHz, at an intensity of 1.0, 0.75, and 0.5 W/cm^2 for 5 and 10 minutes. The ascending rate for 10 minutes was significantly higher than that for 5 minutes, and the descending rate for 1.0 W/cm^2 was significantly higher than that for 0.5 W/cm^2. These resuits suggest there is an optimum ultrasonic therapy of the masseter muscle for temporomandibular disorders.