The purpose of this study was to determine TV and computer games in underweight and obese boys and their relationship with physical activity, fitness and body composition. 482 male students (9 to 15 years old) participated in this study. They were selected from state (n=286) and private (n=196) schools. Subjects were sampled in two stages; at first, cluster sampling was randomly performed and some schools were selected by their geographical location and then some students from different classes were randomly selected. Anthropometric measurements including height, weight and body mass were assessed (149±13.1 cm, 45.32±14.05 kg, 19.85±4.04 kg/m2 respectively). Data of TV and computer games and physical activity level were collected via time diary questionnaire. To determine weight classes (obese, overweight, normal weight, underweight), the Cut off-BMI was used. Fitness level was assessed via AAHPERD test. Statistical analysis was applied using the Kendall's correlation coefficient, Kruskal-Wallis and U-Mann Whitney tests. Results indicated a significant difference in the level of TV and computer games between underweight and obese boys (P<0.05): obese boys spent more time to watch TV and play computer games when compared to underweight boys. Also, the results indicated a significant relationship between TV and computer games with physical activity, lean body mass, percent of body fat and BMI (P<0.05). But there was not a significant relationship between TV and computer games with fitness (P?0.05). Therefore, we can conclude that high level of TV watching and playing computer games results in obese and overweight children and limiting TV and computer games may be useful for preventing children from gaining weight.