The purpose of this study was to clarify the association between age, physical activity and obesity index in men. The total of 226 men, aged from 20s to 60s, participated in this study. We analyzed body composition and physical activity. According to age, muscle mass in 20s was higher than in 40s to 60s and 30s was higher than in 50s to 60s. Body percent fat in 20s was lower than in 30s to 60s. High physical activity in 20s was higher than in 30s to 60s and moderate physical activity in 20s was higher than in 30s to 50s. Low physical activity in 20s was higher than in 40s and sedentary times in 30s was higher than in 60s. Energy expenditure per day in 20s was higher than in 30s to 50s. With increasing age, muscle mass decreases but the percent body fat increased. Also, high, moderate, low physical activity and energy expenditure per day decreased with increasing age. Percent body fat was negatively correlated with muscle mass and muscle mass was positively correlated with high, moderate, low physical activity and energy expenditure per day. In the conclusion, physical activity decreased with age increasing and these changes appear after in 30s. Therefore, we suggested that promoting physical activity plan should be established with age increasing, especially from the early 30s.