1251 It has been reported that players in several sports improve physical capabilities when they use a mouthpiece during their performance. However, the conventional mouthpiece had problems such as discomforts, swallowing depressions and speaking difficulties and there was few report using mouthpiece during swimming. We have devised a mouth piece (soft occlusal plate: SOP) which addresses these problems. We selected polyolefin-based thermoplastic elastomer which has appropriate strength and elasticity as a basic material for a custom made full mouth type mouthpiece. To determine the effects of the SOP, we evaluated swimming performances with or without the SOP. The subjects were fourteen college swimmers. Their age was 20.1 ± 0.10 yr, standing height was 165.0 ± 5.59 cm, body weight was 60.1 ± 6.94 kg. Each subject was informed of the purpose of this study and procedure and their consent was obtained. A set of 15 repeated 100 m free style swims, push-off glide distance from the wall, hand-in distance (entry point length from starting line) and start reaction time with or without the SOP were measured. The mean swimming time was significantly difference between with SOP (75.4 ± 2.09 sec) and without SOP (76.5 ± 2.67 sec). Push-off glide distance and hand-in distance with SOP (8.44 ± 0.75 m, 2.95 ± 0.16 m) was significantly longer than those of without SOP (8.07 ± 0.59 m, 2.89 ± 0.18 m). The start reaction time was 0.87 ± 0.07 sec with SOP and 0.87 ± 0.11 sec without SOP, the differences between conditions was not significant. The use of the SOP resulted in improved speed and decreased resistive force, probably due to the increased muscular strength and maintaining an ideal stream line.