Bone density reaches its peak in the mid-20s, and it manifests as osteoporosis and osteopenia with aging. Bone density is affected by body mass index, muscle mass, nutritional calcium and vitamin D, various lifestyles, physical activity level, and high level of strength. The purpose of this study was to investigate the difference in diet and bone density according to physical activity level in growing male and female adolescents. This study involved 646 male and 581 female adolescents using data from the Korea National Health and Nutrition Examination in 2009-2011. The measurement of bone density consisted of dual-energy X-ray absorptiometry, and they were classified into low, middle, and high groups at different ages based on total bone mineral density. The Korean version of the International Physical Activity Questionnaire by the World Health Organization was used to measure physical activity level, and a survey regarding strength exercise and stretching was conducted. In the nutritional survey, data from a 24h recall were analyzed. One-way analysis of variance and chi-square test were conducted to examine the significance of any differences present. Even though there was no difference among groups in both males and females, there was a significant difference in weight (p<0.05). There was no significant difference among groups by nutrition intake in female adolescents (p>0.05). In males, the high group showed significantly higher calorie intake (p=0.032), protein (p=0.015), calcium (p=0.043), and phosphorus (p=0.013) compared with the low group according to nutrition intake. In terms of physical activity level, there was significant a difference as the proportion of strength exercise more than 3 times a week was 18.1% and 27.2% in the low and high groups, respectively in males (p=0.046), and was 1.0% and 6.1% respectively in females (p=0.014). The proportion of high-intensity exercise 6-7 times a week also showed a significant difference as the low, middle, and high groups showed 5.1%, 5.5%, and 14.1%, respectively (p<0.001). Among adolescents, bone density of female was affected by strength exercise, and that of males was affected by physical activity level and nutrition, showing a clearer tendency in the males. Particularly, regarding physical activity level, high intensity and strength exercise had more positive effects.