Mouth guards are effective to prevent traumatic injury in maxillofacial region during sports activities. This study was conducted to investigate the influence of wearing mouth guards on occlusal force distribution with the T-scan system. With comparison to the value of average occlusal force level per occlusal point (OFL) on natural dentition and wearing conventional full arch bite plane, following results were obtained. 1) Difference of OFL within the same dental arch became smaller when mouth guards were on. 2) Statistically significant difference in OFL was not found between two mouth guards with different hardness. 3) During three seconds clenching, OFL in posterior region changed great deal at the initiation but stayed at about the same level after that.