Objectives: The aim of this study was to evaluate the effect of oral health knowledge and improvement of behavior in elementary school children by Mass Tooth-brushing Exercise for caries prevention. Methods: 30 elementary schools in Tokyo metropolitan and suburban area including 1881 upper grade pupils participated the Mass Toothbrushing Exercise. 485 pupils from 8 schools were randomly chosen for questionnaire elvaluation before and after the exercise. Results: Oral health knowledge and behavior of school children were improved after participation of the exercise. The children understanding of "early stage caries" increased by 31.5% and "dental plaque" by 32.9%. Tooth brushing frequency was also improved from 2.22 to 2.56 times a day in average. With regard to the questionnaire, positive questions seem to lead children to correct knowledge. Conclusions: These results indicated that Mass Tooth-brushing Exercise promoted understanding of oral health knowledge, enhancing oral health consciousness and behavior modification of oral health.