The rate of inactivity among young people in Taiwan remains high. However, few studies have explored physical activity patterns with a longitudinal approach in this population. Based on the revised health-promotion model, this study used 2-wave panel data to test a structural model of how individual characteristics, cognitions, and interpersonal influences predicted physical activity of Taiwanese adolescents. The findings suggest that gender, social support, modeling, self-efficacy, and perceived benefits and barriers to performing physical activity directly and indirectly influence the behavior of physical activity in Taiwanese adolescents. The hypothesized model explained 25% of the variance in physical activity. The implications for parents and policy are discussed.