One primary goal of physical education (PE) is to promote students' lifelong physical activity (PA). This goal implies that PE should not only improve students' PA in PE classes but also promote their PA outside of the school known as the "PE effect." Purpose: In this study, we proposed a two-pathway model of the "PE effect" and hypothesized that learning in PE and positive motivational experience in PE are two possible pathways through which the "PE effect" emerges. We tested the tenability of this two-pathway model from the perspective of knowledge learning and autonomous motivation in PE. Methods: A total of 394 eighth-grade students from five schools provided data on PA knowledge, out-of-school PA, and autonomous motivation toward PE and PA. Structural equation modeling was used to test the two-pathway model. Results: Results showed that the students' knowledge had a direct effect ([beta] = 0.18, p < 0.05) on their autonomous motivation for PA and an indirect effect ([beta] = 0.02, p < 0.05) on out-of-school PA through influencing autonomous motivation toward PA. Students' autonomous motivation for PE had a direct effect ([beta] = 0.41, p < 0.05) on their autonomous motivation toward PA and an indirect effect ([beta] = 0.04, p < 0.05) on out-of-school PA through autonomous motivation toward PA. Conclusion: These results indicate that the two-pathway model is tenable in terms of knowledge learning and autonomous motivation in PE and imply that teaching knowledge in an autonomy-supportive PE environment may be an effective way to promote students' out-of-school PA.