This study examined the measurement structure, cross-year stability of achievement goals, and mediating effects of achievement goals between self-efficacy and math grades in a national sample of Taiwan middle school students. The measurement model with factorial structure showed good fit to the data. In the panel data (N?=?343), four achievement goals showed strong measurement invariance, suggesting factor loadings and intercepts of the items remained invariant across a year. Though mean scores of the four latent achievement goals held quite stable, the rank order of students across two time-points changed more profoundly in the two avoidance goals than in the approached goals. In the cross-sectional data (N?=?748), we found approach-based goals were positive mediators between self-efficacy and math grades while avoidance-based goals were negative mediators. This result could be relevant for middle-school students in learning mathematics. Some instructional implications are provided.