Abstract BACKGROUND: With reference to the importance of physical activity during childhood and adolescence in their health promotion and prevention of chronic diseases during adulthood, it is necessary to have a reliable and valid method for evaluating physical activity level in this age group. Physical activity questionnaire child (PAQ-C) and adolescents (PAQ-A) have been designed for this purpose. This study aimed to validate these questionnaires in a representative sample of Iranian children and adolescents. METHODS: After translating the PAQ-C and PAQ-A to Persian, the face and content validities of these questionnaires were assessed by a committee of experts of different specialties including specialists in pediatrics, community medicine, social specialist and sport medicine. After minor modification of some items of the questionnaires according to the available types of physical activity and the socio-cultural conditions, the final questionnaires were prepared. Thereafter, 82 children and adolescents with 8 to 18 years of age completed the questionnaires. RESULTS: To assess the questionnaire reliability Iranian students and the validity accreditation was modified and got from the committee. A Cronbach’s alpha of 0.894 was produced at the end of the analysis showing it has been reliable for this population. CONCLUSION: According to the results found in this survey, PAQ questionnaire can be used as a reliable and valid method for next studies; so health planning based on Iranian students’ physical activity will be possible in future. Keywords: Physical Activity Questionnaire, Reliability, Validity.