Purpose: The current study explored physical activity and its influencing factors in a random sample of community adults, and tested causal relationship among psychological, social, and physical environmental variables to explain physical activity. Methods: A total of 812 adults over 30 years old who reside in Gangbuk-gu, Nowon-gu, and Seongbuk-gu, Seoul were voluntarily participated in the study. Standardized scales were used to measure physical activity and its related social ecological variables. Results: The findings indicated that physical activity was significantly correlated with all of the social ecological variables. All of psychological, social environmental, and physical environmental variables had a significant direct path to explain physical activity. In addition, self-efficacy and perceived benefits significantly mediated the relationship between family support and physical activity, and while the relationship between availability of facilities and physical activity was substantially mediated by perceived barriers. Conclusion: This study provides useful information in developing and implementing intervention strategies that promote greater engagement in physical activity among middle aged adults.