This study examined the relationships between opportunity to learn (OTL), science engagement, and science achievement in students' middle school level. This study used the Trends in International Mathematics and Science Study (TIMSS) data from the 2003 wave. The data were analyzed using structuring equation modeling and hierarchical linear modeling. It was hypothesized that students' engagement in science is a mediator between opportunity to learn and science achievement. Moreover, class and school level variability was also examined since the organization of the data was nested. The study examined the effects of OTL on students' emotional, cognitive, and behavioral engagement in science and subsequently on science achievement controlling for family socioeconomics status. The results of structural equation modeling supported some theoretical formulations of the conceptual model, and showed significant effect of OTL factors on students' science engagement, especially the behavioral engagement. Furthermore, science emotional and cognitive engagement showed positive effects on science achievement, but the effect of behavioral engagement on science achievement was complex. Detailed exploration and discussions were included in this study. The findings from hierarchical linear models suggested that students' science achievement was not only related to students' engagement, but also varied by class and school level OTL factors. The study had both theoretical and practical significances, providing valuable insights for the pedagogy of science.