The study sought to find out the leadership skills possessed by Principals of public secondary schools in south western Nigeria and the relationship between these leadership skills and school effectiveness in terms of student academic achievement. The descriptive survey research design was employed for the study. 154 Principals and 770 teachers, who were purposively selected, participated in the study. Findings revealed that secondary school principals in south western Nigeria possessed technical, interpersonal, conceptual and administrative skills. A significant relationship was found between principals' leadership skills and school effectiveness. The level of adequacy of possession of leadership skills for school effectiveness was not examined. Training for possession and exercise of principals' leadership skills at sufficient levels to influence school effectiveness was recommended. Within school barriers to the influence of principals' leadership skills on school effectiveness were also recommended for removal.