The purpose of this study was to investigate the relationship between perceived autonomy support, sport motivation and the level of physical activity among male junior high school students in Tehran with the mediating role of autonomy motivation and fear of failure. This descriptive-correlational study is in the category of applied research. The statistical population of the study was comprised of all male seventh graders in Tehran (N=142554). The sample included 384 students which was selected based on random cluster sampling method. The research instruments comprised Autonomy Motivation Questionnaire (Hagger et al., 2007), Black and Deci Autonomy Support Perception Questionnaire (2000), Sport Motivation Scale (Pelletier et al., 2013), Conroy et al.'s Inventory of Fear of Failure (2002), and Measurement of Habitual Physical Activity Questionnaire (Baecke et al., 1982). Structural equation modeling was utilized to analyze the data. Results revealed that teachers’ autonomy support had a direct, positive effect on sport motivation and students’ amount of physical activity. Autonomy motivation and fear of failure play a mediating role in the link between teachers’ autonomy support, sport motivation, and the amount of physical activity. Therefore, physical education teachers can use autonomous supportive behaviors to motivate students to make decisions and thus reduce students' fear of failure and increase students' sport motivation to engage in physical activities.