The purpose of this study was to develop a children's self-efficacy scale for physical activity (CSSP), and to identify the determinants of habitual physical activity. In Study1, the subjects were 455 elementary school children (228 boys and 227 girls) in Grade 4 though 6. The pupils completed a battery of questionnaires in the classroom setting. Results of a principal component analysis indicated that the CSSP had a single-factor structure consisting of 8 items. Four hundred and eleven elementary school children (Grade 4 through 6 ; 207 boys and 204 girls) participated in Study 2. A multiple discriminant analysis revealed that “self-efficacy for physical activity” and “likes and dislikes of physical activity” were the strongest predictors of habitual physical activity for both boys and girls and that “parent's support of physical activity” and “parent's exercise habits” were a moderate predictor for boys. These results indicate that increasing self-efficacy for physical activity and teaching the fun aspects of physical activity to children may promote more habitual physical activity in school and at home.