Relationship of Psychosocial Variables with Physical Activity Participation in Adolescents. The Journal of Kinesiology. 2017, 19(4): 37-46. [PURPOSE] The purposes of the present study were to explore level of physical activity in adolescents, to identify the differences in psychological and social variables by stage of physical activity, and investigate the relationship of psychosocial variables with physical activity. [METHODS] A total of 561 adolescents (male: 229, female: 332, mage = 13.32years) in the Nowon district were voluntarily participated in the study. Stage of physical activity measure, self-efficacy measure, exercise attitude scale, social support measure, and self-regulation scale for physical activity were applied to evaluate the participant s level of physical activity, psychosocial variables relating to physical activity. Frequency analysis, X2 -square test, MANOVA, and regression analysis were conducted to analyze data obtained in the study. [RESULTS] 70.5% of participants reported that they are inactive (precontempolation and contemplation) or doing physical activity on an irregular basis. Moreover, self-efficacy, exercise attitude, internal motivation, and parental support were significantly different according to stage of physical activity. These psychosocial variables had 62% of variance in explaining physical activity and of them self-efficacy was best predictor to account for physical activity. [CONCLUSION] The present study suggests that further study should apply structure equation model to test causal relationship between the psychosocial variables and physical activity.