The purpose of this study was to examine the predictive effects of inquiry-based and teacher-directed instructional practices on students' attitudes toward science and science achievement in a nationally representative sample of 7,708 15-year-old-students from Taiwan who took part in the sixth cycle of the Program for International Student Assessment (PISA). The results of structural equation modeling analyses, after accounting for student demographic and socioeconomic characteristics, revealed that teacher-directed instructional practices had a significant positive effect on students' science achievement, whereas inquiry-based instructional practices had a significant negative effect on students' science achievement. Moreover, the results of the study indicated that inquiry-based instructional practices had greater positive predictive power than teacher-directed instructional practices for students' attitudes toward science. The inverse relations between inquiry-based science instructional practices and student science achievement contradicts the popular belief that inquiry-based science instructional practices help improve student science achievement. However, the results of the current study underscore the positive effects of inquiry-based instructional practices on students' attitudes toward science. Indeed, optimizing the frequency of instructional practices in science classrooms to promote the desired educational outcomes deserves further discussion.