Background: Childhood obesity is currently one of the major concerns in pediatric field. In recent literature, correlation of obesity with some behavioral factors such as physical activity (PA) received attention. Objective: The purpose of this study was to determine the relationship between body mass index (BMI) and PA. Materials and Methods: This cross-sectional study was carried out in 2012 on 548 randomly selected 7-to 11-year schoolchildren from Zahedan, Iran. The underweight, normal weight, overweight, and obesity status were all based on BMI percentiles for age and sex. Moderate to vigorous PA during the previous 7 days was assessed by a modified version of the Physical Activity Questionnaire for Children. Results: The prevalence of underweight, normal weight, overweight, and obesity among the participants was 18.3%, 47.9%, 11.8%, and 22.1%, respectively. Overweight and obesity were significantly higher among girls than boys. Mean PA score in participants was 2.4 + 0.5. The results showed the PA score was significantly higher among boys than girls. The PA score had a significant association with BMI (P ¼ .018), and it was greater in normal participants than in obese participants. Conclusion: On the basis of observed data and due to the impact of PA on BMI as an indicator of nutritional status and health, teaching children to have better nutrition and lifestyle changes is very important. Therefore, more studies should focus on childhood obesity prevention programs to prevent nonoverweight children from becoming overweight or obese in the future.