Sleep bruxism (SB) has been believed to be related to stress and psychosocial factors, however their implicit relationship has remained unclear. This study was conducted on patients visiting our clinic with SB and masticatory muscle disorders (MMD) for the purpose of clarifying personality and behavioral traits. This study was conducted on patients of MMD visiting our clinic. The Rosenzweig Picture-Frustration study was performed on each patient. Twenty-seven (27) patients were divided into two groups: 17 patients with SB and 10 patients without. The SB group showed a significantly lower level of E (extrapunitive) reaction than the nonSB group. SB patients showed a significantly higher level of M (impunitive) reaction than those without SB. Concerning the directions of aggression, the percentage of E-A (extraaggression) was significantly lower in SB patients than in those without. On the other hand, the percentage of I-A (intraaggression) was significantly higher in patients with SB than those without. Our study found a new aspect of the patients with MMD and SB: they are not only intraaggressive, but are also unable to be extrapunitive and extraaggressive. Consequently, they are unable to demonstrate adequate self-assertiveness in stressful situations.