Objectives: The purpose of this study was to evaluate the effect on oral health behavior in elementary school children by Mass Tooth-brushing Exercise for caries prevention. Methods: Invitation letter to participate in the Mass Tooth-brushing Exercise was sent to schools in Tokyo metropolitan and suburban area. The decision to participate the Exercise was made by the staffs of each school such as principals, health education teachers, and school dentists. The number of participating and non-participating schools of each year were 1999 (12, 89), 2000 (12, 93), 2001 (14, 72). The importance of oral health and the way of tooth-brushing was instructed to the children and staffs of participating schools. Caries prevalence in 12-years-old-children (6 grade) and questionnaire about oral health behavior were examined for both participating and non-participating schools to the Mass Tooth-brushing Exercise. Results: Participating school children had lower DMFT (0.8, 0.5, 0.3) and higher treatment rate of dental caries (12.5%, 6.2%, 5.6%) comparing with non-participating schools. Tooth-brushing rate after lunch was also higher (12.8%). No significant differences of dental plaque disclosing activities were found in both schools. The schools participating more times were found to have better oral health condition. The schools with three times of participation had lower DMFT (0.09) and higher treatment rate of dental caries (2.5%) than the schools of less than three times in 2001. Conclusions: 1. The participating school children had better oral condition and behavior than the non-participation school children. 2. The Mass Tooth-brushing Exercise was presumed to increase oral health consciousness of the school children and staffs.