Based on self-determination theory, the purpose of this study was to explore the mediating role of autonomous motivation in the relation between environmental factors and pedometer-determined PA among 10- to 12-year-old Flemish children. Data were collected from 787 6th grade pupils and one of their parents. Children completed self-report measures including autonomous motivation for PA and perceived autonomy support for PA by parents and friends. Parents completed a questionnaire concerning their PA related parenting practices (logistic support and explicit modeling) and the perceived home environment with respect to PA opportunities. The results confirmed that autonomous motivation mediated the relation between children's PA and their perceived autonomy support by friends and parents. Autonomous motivation also mediated the relation between parental logistic support and PA. In addition, a positive direct relation was found between parental explicit modeling and children's PA, and between perceived neighborhood safety and children's PA.