The purpose of this research was to investigate the effect of concept mapping on achievements of students' in Chemistry subject in high schools. In this study, 38 female and 122 male students and 6 Chemistry teachers were participated from two high schools. Since the nature of this research was an experimental, the sample students were divided into experimental and control groups. The experimental groups, followed concept maps approach with mini lecture, while control groups followed the traditional lecture method. The experimental and control groups were exposed to the same study unit that actually covered in the chemistry class.To measure differences between two groups before and after treatment, identical pre-test and post-test evaluations were administered simultaneously. In both secondary schools, there was no significant difference observed in the pre-test scores whereas, statistically significant difference was observed with two sample z-tests (at P>0.05) taken on the post-test scores of students. The correlation between students achievement versus concept map scores were investigated and the result shows there is a significant correlation in both secondary schools.