The study explored Taiwanese students’ mathematics learning at the junior high school level. Utilizing structural equation modeling, it examined the relationships between math self-concept and mathematics achievement with longitudinal data. Participants included 1,256 Taiwanese seventh graders in the first wave and declined slightly to 1,211 eighth graders in the second wave. Findings indicated the following. First, the longitudinal effects were all significant: (a) prior mathematics achievement significantly predicted subsequent math self-concept (skill development model), (b) prior math self-concept significantly predicted subsequent mathematics achievement (self-enhancement model), and (c) the reciprocal effects model was supported, and the effects of achievement tended to become stronger and more systematic. Second, results showed significant gender variation with respect to math self-concept and mathematics achievement. Boys had significantly higher math self-concept than girls, whereas girls exhibited higher mathematics achievement than boys. The implications of these findings for cultivating students’ interest in mathematics learning were discussed.