Any movement of the body produced by skeletal muscles that requires energy expenditure, and muscle workwhich called physical activities. Increased BMI occurs because of an energy imbalance between food intake with energyreleased to cause the buildup of energy in the form of fat that can lead to obesity or increased BMI.This study purposewas to determine the relationship between physical activities and body mass index (BMI) in adolescents aged 16-18years old in SMA Negeri 2 Denpasar. This research was analytical research with cross sectional approach done inFebruary-March 2018. Sampling was done by Simple Random Sampling. Sample amounted to 70 people. The resultsshowed the relationship between physical activities with body mass index. In the calculation of chi square data analysiscould be seen that significant value (2-tailed) was 0,000 which meant that significant value was p < 0.05. Based on theresults of this study, it can be concluded that there is a strong relationship between physical activities and body massindex (BMI) in adolescents aged 16-18 years old in SMA Negeri 2 Denpasar. Keywords : Physical activity, Body Mass Index (BMI)