This study investigates the effects of physical activity, the knowledge level about obesity, watching food ads on TV and school canteen usage on body mass index (BMI) for male (1227) and female (773) children in the middle and high schools. The results demonstrated that body height and weight, and BMI significantly differentiated with respect to age and gender groups. The mean body weight and BMI in exercise group were lower than sedanters. The mean body weight changed depending on knowledge level about obesity. The group watching food ads on TV has higher averages than sedanters in the body weight and BMI. School canteen usage has no effect on the body weight and BMI.