Background: Assertiveness is a vital skill in contemporary health care and an essential component for positive and productive working relationships between nursing staff. Aim of the study: This study aimed to assess the effect of an assertiveness training program on psychiatric mental health nurses' communication skills and self-esteem. Research design: A quasi-experimental research design. Setting: The study was carried out at the Psychiatric Mental Health Hospital in Benha City, Kaluobia Governorate. Sample: A convenience sample of (50) psychiatric mental health nurses was included for the conduction of this study. Tools of data collection: The study tools divided into four tools. I: Structured Interview Questionnaire. II: Rosenberg's Global Self-esteem Scale. III: Assertiveness skills scale and IV: Communication Skills Inventory. Results: There was a highly statistically significant differences between total mean scores of self- esteem, assertiveness skills, and communication skills among the studied sample post program implementation compared to preprogram implementation, also there was a highly statistically significant positive correlation between total self-esteem and total assertiveness skills and total communication skills post program implementation. Conclusion: Assertiveness training program have a positive effect on communication skills and self-esteem of psychiatric mental health nurses. Recommendations: Generalization of assertiveness training program for all newly graduated psychiatric nurses in all psychiatric hospitals