The purpose of this study is to examine the issues of students' motivational beliefs in science learning and their relations with science achievement. Data of Taiwanese fourth and eighth graders from the Trends in International Mathematics and Science Study 2011 were examined. Students' self-concept and intrinsic interest as motivational beliefs, and science scores in general and in different cognitive domains were used as measures in this study. A series of quantitative analyses along with statistical techniques accounting for the properties of international large-scale assessment data were utilized to investigate these issues. The results indicate that students' motivational beliefs decrease over grades. The relations between the eighth graders' motivational beliefs and science scores are positively stronger than those of the fourth graders. Meanwhile, students' self-concept has a greater predictive power than intrinsic interest for science scores in both grades. Beyond documenting students' motivational beliefs over grades, the conclusion also sheds light on the role of motivational beliefs, self-concept in particular, in explaining students' science achievement.