Introduction: Sexual changes in breast cancer occur after diagnosis and treatment include mastectomy. Sexual assertiveness is an effective factor in sexual satisfaction that means the ability to convey sexual feelings, beliefs, and thoughts and logically defend sexual rights. The present study aimed to compare the effectiveness of individual sexual counseling using the BETTER model with individual sexual counseling using the PLISSIT model in terms of increasing sexual assertiveness.Materials and Methods: The present quasi-experimental intervention was conducted in Mashhad in 2021, and 78 mastectomized women with breast cancer were randomly assigned to two groups: BETTER and PLISSIT. Both groups received four individual counseling sessions of 60-90 minutes one week apart. The research tools included demographic information form, female sexual function index (FSFI), and the Hulbert index of sexual assertiveness (HISA). Changes in the mean scores of sexual assertiveness between the two groups were evaluated before and 4 weeks after the intervention, and the mean changes were compared between the groups. Data analysis was conducted using the Kolmogorov-Smirnov test, independent t-test, paired t-test, and Chi-square tests using SPSS 25 (P<0.05).Results: The mean scores of sexual assertiveness were not significantly different between BETTER (46.7) and PLISSIT (43.07) groups before the intervention (P=0.253); however, the mean scores of sexual assertiveness in the BETTER group (54.8) and the PLISSIT group (48.6) were significantly different 4 weeks after the intervention. (P<0.001)Conclusion: The results indicated that sexual counseling based on the BETTER model was more effective in increasing the sexual assertiveness of mastectomized women with breast cancer than counseling by the PLISSIT method.