This paper investigates the relationship between eighth-grade students’ achievement and self-perceptions in mathematics and science by analyzing the three waves of the Trends in International Mathematics and Science Study (TIMSS) data. A total of three measures on self-perception were used, namely, how much students like the two subjects, their self-perceived competence in the subjects, and their perceived easiness of the subjects. For within-country data, with individual student as the unit of analysis, there is generally a positive correlation between students’ achievement and their self-perception. But when the three self-perception measures are aggregated at the country level, the relationship is reversed. In other words, there is a negative correlation between self-perceptions and achievement on a between-country analysis with country as the unit of analysis. This pattern is consistent in both mathematics and science across all three waves of data, even though the sample sizes (number of countries) and the participating countries vary from wave to wave. One possible explanation for this finding is that high-performing countries have higher academic standards; their students have higher pressure to get into top-choice academic institutions by excelling in public examinations. Accordingly, students from these countries have better academic performances in science and mathematics on the average, but lower preference for these subjects.