Background: Head nurses have a crucial leadership role in managing their units and providing high-quality and safe nursing care.  Head nurse leadership competency and effectiveness are very essential to manage nursing care practices and management activities in their hospitals. Development leadership competency intervention programs would improve the head nurse competency and effectiveness. This study aimed to evaluate the impact of the leadership competency intervention program on the leadership effectiveness of head nurses. Subjects and Method: Study design: A quasi-experimental design.  Setting: The study was conducted in inpatient care units (general wards and toxicology, intensive and critical care units) at Alexandria University hospitals. Subjects: The study was carried out on 90 head nurses who participated in the intervention program. Tools of data collection:  Self-assessment questionnaires of leadership competency and leadership effectiveness were used to collect data in this study. Results: The results indicated an enhancement in head nurses' leadership competency and effectiveness levels post and follow-up intervention program.  Overall head nurse leadership competency and effectiveness revealed significant differences throughout the post (Z= -6.051 and   -5.962 respectively) and follow-up intervention (Z= -6.047 and -5.877 respectively) compared to pre-intervention program. The post and follow-up significant correlation (r=.90 and. 94, respectively) and regression results (B=.61, R² =. 81; B=.62, R² =. 88) confirmed that the leadership competency intervention program was effective in increasing and enhancing the leadership effectiveness levels among head nurses. Conclusion: A leadership competency intervention program for head nurses enhanced their levels of leadership competency and effectiveness. Recommendations: Leadership competency intervention programs should be created in healthcare settings to prepare head nurses for leadership competencies, to achieve their management role effectively and support them to become effective nurse leaders.  The necessary resources should be provided for establishing such programs.