Bullying impacts on the development and learning achievement in adolescent. The preliminary survey showed that students have committed acts of bullying like physical, mental, and cyber-bullying. This study identified the effect of assertive behavior therapy on changes in bully behaviour on adolescents. The research design was a pre post-test control group design. The subjects were 40 students which was divided into two groups (20 students in the control group and 20 students in the treatment group), and chosen by simple random sampling. The data collection used The Aggression Bullying Scale. The data analysis used descriptive analysis; paired t-test and independent t-test with SPSS 20. The data showed differences in pre-post test scores in the treatment group with p=0.000 (α≤0.005) and there was a differences in post-test scores in the control and treatment groups with p=0.000 (α≤0.005). The result showed that Assertive Behavior Therapy reduced bully behaviour in adolescents.