Physical movement is reduced because of convinient life style by advancing of science. People need physical movement and athletes also need more physical training for their health and better records because of reduced movement in modern life style. Some athletes competing in contact sports wear dental mouth guards to reduce the risk of orofacial trauma and to increase the strength of muscle. It has been speculated that the use of mouth guards improves athletic ability such as muscular strength and equilibrium. The purpose of this study was to test the influence of C.M(custom made) mouth guard according to strength of taekwondo athletes. 5 trained subjects participating in taekwondo students in D University were included in this study. Deltoid muscular activity were tested by means of electromyograph (E.M.G) with or without wearing mouth guard. Subjects have pressed with 5 Kg barbell. The data were transferred to computer system as raw and integral data. The data were analysed by computer system, showing wearing mouth guard had higher action potential than non-wearing mouth guard. This meant that wereing mouth guard could enhance the muscle force in deltoid. It suggested that taekwondo athletes could use C.M mouth guard without any negative effect on their muscle strength.