Objectives This study investigated the effectiveness of Teaching Games for Understanding (TGfU) intervention on the moderate-to-vigorous physical activity (MVPA) levels of students from Grades 9 and 10. Methods A total of 118 students from four classes (two TGfU groups and two technique groups) participated in this study. Accelerometers were used to measure the MVPA time of students, and formal interviews were conducted to identify the factors that contributed to the MVPA level of students in TGfU classes. Results The results reveal that the TGfU [t (1, 57) = - 11.622, p <.001, and d =.841] and technical [t (1, 61) = -4.232, p <.001, and d =.236] group exhibited significantly improved MVPA levels in intervention phase. During the intervention period, the MVPA time of the TGfU group (M = 20.26, SD = 3.74) was significantly longer than that of the technique group [M = 17.62, SD = 3.37, t (1, 118) = 4.023, p < .001, and d = .35]. Moreover, in the TGfU classes, boys (M=21.476, SD =.719) spent significantly more time engaging in MVPA than girls (M = 19.135, SD = .645) [F (1, 57) = 5.807, p = .019, and η2= .09]. However, no significant differences were determined between the MVPA levels of high- and low-skilled students. Data gathered through interviews suggested that the nature of the games, the small-sided team, and the freedom and enjoyment experienced by the students through games may explain the high MVPA levels observed in the TGfU classes. ConclusionTGfU intervention can potentially be used to promote physical activities and attain the recommended MVPA time in PE classes (50% class time).