Assertiveness is one of the essential skills in the modern working environment, there are many benefits of being assertive such as; better time management, increased self-esteem and the ability to negotiate more effectively. Psychiatric mental health nurses are confronted with the daily responsibility of assessing, intervening and evaluating client responses to stress and client interactions. They need to identify the meaning of their own stress and develop strategies that increase personal and professional growth. Some nurses may have learned to express their needs honestly to use anger constructively, to improve personal and work relationships and to build self-confidence through assertive behavior. But others still struggle with being assertive, perhaps because they don’t believe they have the right to be assertive, they are anxious about asserting themselves or they don't know how to be assertive. Learning assertive techniques are considered as tools for coping skills with a variety of life stressors. Increasing assertive behavior becomes an expression of person's feelings. So the aim of this study was to assess the effect of assertiveness communication skills training program on improving self-esteem of psychiatric nurses. Subjects and methods: The study was conducted at The Psychiatric mental health Hospital in Assiut, Egypt. The study sample comprised 30 nurses (14 males and 16 females), three tools were used for data collection, namely: Sociodemographic Data Structured Interview schedules, , Self – esteem inventory, assertive Communication skills checklist (ACs – list),. The main results yielded by the study proved that, the highest percentage among nurses aged from or more than 30 years, females than males, and single, more than ⅔ (73.3%) of the studied group was lived in rural area. the study recommended continuous education provided for all nursing personal working and make nurses updated of new knowledge and technology. Conclusion: Implementation of assertiveness training program with psychiatric nurses has a positive effect on improving their self-esteem. Recommendation: Implementation of assertiveness training program for all nurses and for undergraduate nursing students.