Purpose: This study aims to examine the effectiveness of a whole-of-school approach by using the 4PC model (Active Policy, Active People, Active Program, Active Place, and Active Classroom) in improving physical activity and reducing sedentary behavior of school children in Thailand. Method: We employed a quasi-experimental cohort design in which the intervention group was exposed to the 4PC model and control schools performed their regular routine. We followed the same students from 10 participating schools over a 2-year academic period (2017-2019) from primary school Grades 4-6. A total of 119 of 184 students in the intervention group, and 173 of 254 students in the control group were present in all five rounds of data collection and are included in the analysis. Results: Compared to students in the control group without the 4PC exposure, students in the intervention group accumulated an additional 19-25 min of physical activity time and experienced a 31-min reduction in sedentary time. Conclusion: As a whole-of-school approach, the 4PC model was effective in increasing physical activity and reducing sedentary behavior of primary school children in Thailand.