The main purpose of the present study was to investigate the effect of case-based instruction on remedying 10th grade students' alternative conceptions related to gas concepts. 128 tenth grade students from two high schools participated in this study. In each school, one of the classes was randomly assigned as the experimental group and the other class, instructed by the same chemistry teacher, was assigned as the control group. The students in the experimental groups were instructed by case-based instruction based on conceptual change conditions while the control group students received traditionally designed chemistry instruction. As pre-tests, the science process skills test, the attitude and motivation towards chemistry and the gas concept test were applied to both groups of students. As a post-test, the gas concept test was administered to both groups of students to determine their alternative conceptions and understanding of gas concepts. One-way ANOVA was used to assess the effect of case-based instruction on students' understanding of gas concepts. The results revealed that case-based instruction was an effective method for overcoming students' alternative conceptions about the gas concepts.