The purpose of the present study was to investigate the differences of EMG activity of the masticatory muscles between normal occlusion and Class III malocclusion during various jaw functions. 46 subjects of 18.4-25.7 years were employed in this study: 26 subjects were normal occlusions, and 20 subjects were Class III malocclusions. The EMG data from the anterior and posterior temporal, anterior and posterior masseter muscles in both sides as mandibular elevators and supra-hyoid muscle group (close to the anterior belly of digastric muscle in right side) as mandibular depressor were recorded with the Medelec MS 25 electromyographic machine. The EMG recordings were analyzed during mandibular rest position, maximal biting, mastication with chewing gum, and swallowing of peanuts. All data were recorded and statistically processed. 1. The maximal mean amplitude of the anterior temporal muscle was stronger significantly in Class III malocclusion than in normal occlusion, and then the posterior temporal was weaker during mandibular rest position. 2. The maximal mean amplitudes in the anterior and posterior temporal muscles and the anterior masseter muscle of Class III malocclusion was weaker significantly than that of normal occlusion during maximal biting. 3. During mastication of the chewing gum, the maximal mean amplitudes of Class III malocclusion was weaker significantly than normal occlusion in the anterior and posterior temporal muscles of the working side, and the duration of Class III malocclusion was longer in the anterior temporal muscles of both sides, and the posterior temporal and the anterior masseter muscle of the balancing side. There were significant increasings of the latency in balancing anterior temporal, working posterior temporal muscles and supra-hyoid muscle group of Class III malocclusion. The silent period durations was 16.36 ms in Class III malocclusion while 10.76 ms in normal occlusion, which was statistically different (P<0.05). 4. At swallowing of peanuts, the maximal mean amplitude of Class malocclusion was weaker significantly in the posterior temporal muscle than that of normal occlusion. There was no significant difference of duration between normal occlusion and Class III malocclusion. 5. The muscle activities of Class III malocclusion had a tendency of decrease less than normal occlusion. And then the muscle activities of the anterior temporal and anterior masseter muscles in Class III malocclusion showed the tendency of the increase more than other muscles of Class III malocclusion.