It is widely accepted that adolescents’ physical activity (PA) level declines with increasing age. A scrutiny of national surveys, however, reveals the stability of moderate and vigorous physical activity (MVPA) during middle school ages (CDC, 1996). The present study was intended to further examine six variables of PA among sixth, seventh, and eighth graders and identify the possible factors, if any, contributing to the maintenance of middle school students’ PA level. A total of 138 middle school students (34 sixth graders, 14 girls, 20 boys; 55 seventh graders, 34 girls, 20 boys; 49 eighth graders, 24 girls, 25 boys) in the US participated in the study. The Self-Administered Physical Activity Checklist (Sallis et at., 1996) was administered to the participants. The information regarding participants’ current involvement in organized sports was obtained with a survey finished by participants and PE teachers. While a 2 × 3 (gender × grade) MANOVA did not reveal the significant differences (p > .05) in PA level among the three graders, a obvious increase in PA level with the increase of grade was observed, especially between sixth grader and the other two graders, in each of the six PA variables: (a) minutes of PA (M for sixth, seventh, and eighth graders was 78.79, 105.05, and 101.24, respectively), (b) minutes of MVPA (66.82, 70.05, 68.27), (c) MET score (6.92, 9.11, 9.10), (d) MVPA MET score (6.23, 7.13, 7.22), (e) weighted MET score (7.20, 9.66, 9.90), and (f) weighted MVPA MET score (6.51, 7.55, 7.83). Moreover, the results of Chi Square test indicated a significant increase (χ² = 8.35, p = .015) in the involvement in organized sports when children got older: 17.6% of sixth graders, 29.1% of seventh graders, and 46.9% of eighth graders. The results suggest the stability and increase in PA level in middle school period. Middle school children, still young enough to have an instinct for enjoying PA, begin to be exposed to a wide variety of formal sports, and explore and find the sports/activities that interest them. With their increasing competence in sports skills and getting more involved in organized sports, the chance for them to maintain, even increase, PA level exists. It seems that the middle school period is a good time for PE teachers to develop students’ habitual PA participation habits.