This study investigated whether students' motivational beliefs about learning science (i.e., self-concept in science and intrinsic value of science) varied significantly across grade levels and genders. The study also examined the relations between students' motivational beliefs about learning science and their science achievement by grade level and gender. The data for the study included the Taiwanese samples of students at grades 4 and 8 who took part in the 2011 and 2015 Trends in International Mathematics and Science Study (TIMSS). The results of the independent samples t-tests indicated that mean scores on the science motivational beliefs scales decreased significantly from grade 4 to 8. Boys' science motivational beliefs were significantly stronger than girls' science motivational beliefs in both grades, and the differences in mean scores between boys and girls increased from grade 4 to 8. Moreover, the results of the hierarchical regression analyses suggested that the strengths of the associations between science motivational beliefs and science achievement increased from grade 4 to 8. The relationships between science motivational beliefs and science achievement were significantly weaker for girls than for boys in grade 4. The findings of the study highlight both gender and grade level differences in motivational beliefs about learning science and their relations with science achievement.