Background. Regular physical activity is associated with positive consequences for physical and mental health of children. Previous studies used self-determination theory, mostly based on the need for autonomy, for creating interventions in the physical education class for improving the level of physical activity among students. The purpose of this study was to further investigate this issue by designing an intervention based on the need for competence. Thus, we examined the aim of a competence-based intervention in the physical education class on improving the level of physical activity in high-school students. Research methods. Participants included 198 high-school students (mean age of 16.87 years) who studied in regular schools in Iran. Participants were randomly assigned into intervention and control groups. Physical education teachers of the intervention group attended in a training course consisting of several strategies to support need for competence within physical education class. The experimental procedure consisted of the pretest and posttest. Intervention program lasted for four months. We used accelerometers to assess PA pattern during the physical education class. Other variables such as perceived competence support, needs satisfaction, and motivation were measured by using standard questionnaires. Structural equation modelling by using Lisrel software was employed to analyze the data. Results and discussion. Results showed that competence-based intervention enhanced significantly perceived competence support, basic needs satisfaction, and autonomous motivation from pretest to posttest and follow-up. Moreover, moderate-to-vigorous PA was significantly increased from pretest 15.49%) to posttest (27.74%) in the intervention group. Finally, no significant improvements were observed in the control group regarding all research variables. Conclusions. School-based interventions might be an appropriate strategy for enhancing motivation and PA among students.