Purpose: This study was done to develop an applicable training program for high school girls on sexual assertiveness. Method: The design combined methodological study and non-synchronized quasi experimental study designs. The participants were 174 high school girls from two schools. Two questionnaires were used, one consisted of 23 questions on self efficacy and the other, of 22 items on sexual assertiveness. The SPSS 10.0 program was used for data analysis. Experimental group 1(3 hour program) and experimental group 2(6 hour program) were received on sexual assertiveness. The sexual assertiveness program was carried out by members of the research team. There were no differences in demographic characteristics between control group, experimental group 1, and experimental group 2. Result: There was no significant increase in self efficacy scores in experimental group lover the control group, but there was a significant increase in sexual assertiveness scores in the experimental group lover the control group. There was an increase in self efficacy scores in experimental group 2 over the control group, but it was not significant. There was a significant increase in sexual assertiveness scores in experimental group 2 over the control group. There was no significant increase in self efficacy scores in the experimental group 2 over the experimental group 1, and there was no significant increase in sexual assertiveness scores in experimental group 2 over the experimental group I. Conclusion: The results show that a 3 hour program was as effective as a 6 hour program for sexual assertiveness. Therefore, it is advisable to use a hour program for clinical convenience. But further study is needed to determine the lasting effect on sexual assertiveness.