The purpose of this study is to examine the relationships between fourth and eighth grade students' self-concept and achievement in mathematics and science within one educational system based on the big-fish-little-pond effect (BFLPE) model. The samples were from the data of the Trends in International Mathematics and Science Study 2011, and hierarchical linear modeling was used to differentiate the effects of the student-level and school-level achievement on student self-concept of learning in Taiwan. The results indicated that the self-concept of students of both grade levels is positively associated with individual achievement in mathematics and science. However, there is generally a statistically significant and negative correlation between student self-concept and school mean achievement. At the same time, the results also showed that the correlation was stronger for the eighth than for the fourth grade students, and for mathematics than for science. The findings of this study document the different degree of BFLPE on students of different grades and on different academic subjects.