The first aim of this study was to determine the intensity of physical activity (PA) by an objective measurement method - using an accelerometer in Turkish prepubertal boys. The second aim of this study was to correlate the intensity of PA with body mass index (BMI). For this aim 40 boys were participated in this study. PA levels were measured by using GT3X-bt accelerometer for 5 consecutive school days. Mean BMI was found 17.56 ± 2.39. 101.20 ± 17.82 minutes was found as moderate to vigorous physical activity (MVPA) per school day. Significant negative moderate correlation was found among vigorous PA, weight and BMI (r= -.39; -.38). The main finding of this study is: there is a relationship between vigorous PA and BMI in prepubertal boys. This means that the most effective intensity of MVPA in preventing the increase of BMI in prepubertal boys is the vigorous PA.