This study was performed to measure the mandibular movement and the changes of masicatory and sternocleidomastoid muscle activity reflected by occlusal disturbance during habitual chewing. For this study, 18 subjects(14 males and 4 females with an average age of 24.0) were selected. The impression of each subject were taken for measuring intermolar distance on lower dentition. The activities of masticatory and sternocleidomastoid muscle and the mandibular movement were recorded and analyzed during habitual chewing by means of E.M.G.(electromyograph), E.G.N.(electrognathograph), rotate program in BioPak analyzing system(BioResearch Inc.). The results were as follows : 1. In EMG of the mandibular rest position, the mean value of muscle activites were increased by nocleidomastoid muscle and anterior belly of digastric muscle(0.05<p<0.1) in working side. 2. During habitual chewing the opening and closing velocities were increased(0.05<p<0.1) after removal of occlusal disturbance and it ws decrase 1week later. 3. During habitual chewing the opening velocities were faster than closing velocities in mandibular movement trace. 4. During habitual chewing the degree of mandibular horizontal rotation is increased(p<0.005) by occlusal disturbance and decreased(0.025<p<0.05) after removal of occlusal disturbance. 5. During habitual chewing mesioanterior-downward translation of non-working side mandibular condyle is more limited in anterior-mesial movement than in inferior-mesial movement caused by occlusal disturbance. 6. During habitual chewing the degree of mandibular frontal rotation is more decreased in the subjects of group function guide than in the subjects of canine guide caused by occlusal disturbance.