This study examined the relation between adolescent expectancy-value motivation, achievements, and after-school physical activity participation. Adolescents (N = 854) from 12 middle schools completed an expectancy-value motivation questionnaire, pre and posttests in psychomotor skill and health-related fitness knowledge tests, and a three-day after-school Physical Activity Recall. Data were analyzed using structural equation modeling to test an a priori model. Results revealed that expectancy belief significantly predicted adolescent psychomotor achievement, and that psychomotor achievement was the only direct significant predictor for physical activity participation (p < 0.05). Expectancy belief and task values were not significantly directly associated with adolescent physical activity participation (p > 0 .05). The findings suggested the relation between adolescent expectancy-value motivation and physical activity participation is likely to be mediated by their psychomotor skill achievement.