In order to observe the effect of mandibular orthopedic repositioning appliance (MORA) on body strength, the author measured back muscle strength with Digital Back Muscle DYNAMOMETER (TAKEI KIKI KOGYO Co., Tokyo, Japan) before MORA, 15 days after and 30 days after MORA in 12 men and then analysed them statistically. The subjects were weight-lifting athletes of Seoul Athletic High School without dysfunction of masticatory system. MORA was fabricated at 1.0-1.5rnm isotonically closed position (Myocentric Occlusion Position) from mandibular rest position (Myocentric Rest Position) using Myo-monitor (MyoTronics Research Inc., Seattle, Washington, USA) and-SVT C-II (Tokyo Shizaisha Inc., Tokyo, Japan). The results were as follows : 1. The mean of back muscle strength before MORA was 150 kg, that of 15 days after MORA was 165 kg (4.43% increase than before), and that of 30 days was 175 kg (10.76% increase than before). 2. There was a significant difference among back muscle strength before MORA, 15 days after and 30 days after MORA (P<0.01: repeated measures one-way ANOVA). 3. According to the result of determining mandibular position for MORA fabrication, mandibular rest position after relaxation with Myo-monitor was 3.2mm lower, 1.0mm posterior, and 0.5 mm left from centric occlusion. And the mean of the amount of lateral deviation was 0.7 mm.