Abstract: Objective To provide evidence for diagnosis of unilateral mastication in the early stage by means of the surface electromyography (sEMG) of masseter muscle (MM).Methods 30 subjects with bilateral chewing and 52 subjects with unilateral chewing were measured by means of sEMG.The subjects were instructed to stay at mandibular postural position (MPP) and to make maximum clenching at intercuspal position (ICP) and chewing.The sEMG of left and right masseter muscles were separately recorded.Asymmetry index of masseter muscles (ASMM) was calculated and compared.Results ①ASMM at MPP,during maximum clenching and chewing in unilateral chewing group were significantly higher than those in bilateral chewing group (P<0.01).②There was no statistical difference in ASMM at MPP,during maximum clenching and chewing between the males and the females among bilateral chewing subjects.③In bilateral chewing group,the 95th percentile of ASMM at MPP,during maximum clenching and chewing were 5.66%，10.65% and 12.73%,respectively.Conclusions At MPP,during maximum clenching and chewing,the bilateral symmetry of masseter muscles in unilateral chewing people was obviously less than in bilateral chewing people.

Key words: unilateral mastication, masseter muscles (MM), surface electromyography (sEMG), diagnosis