The Self-determined Exercise and Learning For FITness (SELF-FIT; Australia New Zealand Clinical Trial Registry: ACTRN12615000633583) intervention was designed based on self-determination theory to increase secondary school students' physical activity levels. The intervention included a novel gaming tool (fitness dice) and teachers were trained to teach in an autonomy supportive way. In this study, we reported the qualitative data collected from the trial. Seventy-five students and 11 teachers from 10 Hong Kong secondary schools were interviewed through focus groups and through interviews respectively. Thematic data analysis was conducted. Students and teachers felt that the intervention enhanced students' knowledge of physical fitness and how to perform exercises (competence), gave them control over which activities to do (autonomy), and increased students' connectedness with peers and teachers (relatedness). In turn, students' motivation for physical activity was enhanced. These findings provided evidence for effectiveness of SDT-based PA promotion intervention with autonomy-supportive teachers. Copyright ©Edizioni Luigi Pozzi s.r.l.