This study is based on a secondary analysis of data collection from 5,690 Taiwanese 8th graders in TIMSS 1999. Marsh and Köller’s (2003, 2004) Unification model was examined by using a structural equation modeling technique. Due to limitation of cross-sectional data, the Unification model was further divided into an internal/external frame of reference model (I/E model) and a motivation-resource competition model (MR model). In support of the I/E model, math achievement had a positive effect on math self-concept but a negative effect on science self-concept. On the other hand, science achievement had a positive effect on science self-concept but a negative effect on math self-concept. Consistent with the revised MR model, self-concept in one domain (math or science) had a positive effect on achievement in the same subject, but a negative effect of achievement in another domain (science or math). The results indicated that I/E model and modified MR model could be generalized to math and science subject areas.