Weight concerns and dieting are prevalent among female adolescents both in Western and Asian countries. They can result in negative psychological and physiological consequences. This study aimed to examine the relative importance of social and personal facts on the decision to diet of female adolescent students in Japan, applying the Theory of Planned Behavior. Data were collected from 5 junior high schools and 3 high schools in Naha City, Okinawa Prefecture, in 2010, through self-administered questionnaires. The data of 756 female students were assessed. The independent variables included social factor variables (norms) and personal factor variables (attitude to dieting, perceived behavior control, body esteem, body-figure discrepancy and past dieting). The dependent variables were dieting intention and behavior. Hierarchical multiple regression analyses were performed to examine three models; model 1 (age and obesity index), model 2 (social factor variables with age and obesity index) and model 3 (all variables). Although the model 2 failed to explain a substantial proportion of the variance, the model 3 was able to explain nearly a half of the variance of intention (R 2 = 0.507) and more than one third of the variance for behavior (R 2 = 0.376). Past experience of dieting was the best predictor of both dieting intention and behavior. Body esteem was the second best predictor for dieting behavior. In conclusion, personal factors are more likely to drive the dieting decision, compared to social factors. Diet education programs should consider targeting frequent dieters and those with poor body esteem.