From ancient soldier to mordern athletes, it has been discussed that some oral appliance have enhanced performance and decreased stress or improve strength. Since mouth guard was used by Italien Boxer on 1930’s for prevent of orofacial injury, some investigators have researched the co-relationship between jaw position and strength. This experiment was performed to study the relationship between mouth guard and E.M.G. 20 Taekwondo students were participated in this study. They were wearing Mouth guard and checked Brachialis physical activity by E.M.G. The muscle activity of mouth guard wearing group was higher than that of non-wearing group.