The aim of this study is to investigate the effectiveness of conceptual change oriented instruction (CCOI) over traditionally designed chemistry instruction (TDCI) on overcoming 10th grade students' misconceptions on gases concepts. In addition, the effect of gender difference on students' understanding of gases concepts was investigated. The subjects of this study consisted of 74 10th grade students from two chemistry classes. One of the classes was assigned as experimental group and the other group was assigned as control group. The experimental group was instructed with CCOI and the control group was instructed by TDCI. Gases Concept Test (GCT) was administered to both groups as pre- and post-tests to measure the students' conceptual understanding. The results showed that students in the experimental group got higher average scores from Gases Concept Test. Also, a significant difference was found between the performance of females and that of males in terms of understanding gases concepts in favor of males.