Aim: This study investigates the impact of assertiveness training on assertiveness and self-esteem of high school students. Method: The study employs Quasi-experimental design where 130 participants aged 13-17 years were selected randomly and assigned into two groups as experimental (N-65) and control group (N-65). Tools: For this aim Assertiveness scale (AS) and Self-esteem questionnaire (RSE) were used. Training: During the treatment phase, the experimental group received assertiveness training of 5 weeks comprising two sessions per week, and each session took 45 minutes. After treatment both experimental and control groups are measured with post-tests. Results: The results show that assertiveness training has significantly increased the assertiveness and decreased the aggression and submission in the experimental group. Also the training has significantly increased the self-esteem of students. Conclusion: The obtained findings revealed an increase in the rate of self-esteem and assertiveness and decrease in the aggression and submission of students. Hence it proved that assertiveness training is significantly effective on the assertiveness, aggression, submission and self-esteem of students.