Introduction: Despite the importance of exercise and physical activity on the health of elderly, the amount o physical activity (exercise) among this vulnerable class of society is declining. This study was conducted with the aim of identifying the determinants of the implementation of physical activities in the older adults in Tehran.Method: This Study was a qualitative design and carried out using Grounded theory methodology. Data was collected via semi-structured in-depth interviews and field notes. Data collection was conducted in the year 2015. Thirty elderly and 10 sport experts were interviewed were interviewed. Analyzing of the data was carried out with the Corbin and Strauss (2008) approach.Results: Concepts of the study were identified using micro and macro analysis. In during of data analysis, 350 free codes related to elderly' physical activity were extracted and classified into three categories, 200 of which were assigned to barrios and 150 to motivators. These categories were environmental factors, interpersonal factors, and intrapersonal factors. Conclusion:Compared to quantitative studies, this study managed to identify the determinants of the implementation of physical activities in elderly in a better and deeper way. By identifying the motivators and barriers to the implementation of physical activity in elderly, those in charge of providing sports services to the elderly can facilitate elderly ' participation in physical activities by exploiting the motivators and removing the barriers and improve the physical and mental health of the elderly because of this increased participation.