OBJECTIVE The knowledge, attitude, practice (KAP) studies play an important role in promotion of oral health. The aim of this study was to assess oral health KAP in 12-year-old schoolchildren in Iran. METHODS In a cross-sectional study on 1554 students aged 12 years from the rural and urban areas of five provinces, KAP of subjects was assessed using a culturally adapted questionnaire. Data were analyzed using Chi-squared test, t-test, correlation coefficient, analysis of variance (ANOVA), and linear regression. Statistical significance was set at P < 0.05. RESULTS In general, the scores of students in knowledge, attitude, and practice were 74.1 ± 21.1%, 72.6 ± 21.1%, and 51.8 ± 12%, respectively. Urban students had significantly higher scores in KAP (P = 0.0001). Female students got higher scores in all the three dimensions, but the difference was significant only in brushing practice (P = 0.006). Students in Tehran (the capital city) had significantly higher scores in all fields (P = 0.0001). Parents' education and mothers' job had a significant relationship with KAP of children. CONCLUSIONS The results of this study showed that 12-year-old schoolchildren in Iran had good knowledge and positive attitude. But oral health practice was not satisfactory.