This study depicted a micropolitical analysis of school principals' decision making as regards the influence of formal and informal groups on school administrative processes from the point of view of principals. It was based on descriptive survey study of all 24 public secondary schools within Ile-Ife community, Osun State, Nigeria, out of which a sample of 10 schools was purposively selected. The instrument for data collection was an open-ended questionnaire titled "The micropolitics of school principals' decision making in Nigeria: Principals' perspective". The results showed that decisions' themes focused on improving quality of teachers and physical facilities in schools. The formal groups responsible for these decisions were principals, teachers, government officials, and parents. The informal groups were watchmen or night guards, non-governmental organizations, mass media agencies, students, and landlords. The study concluded that a complex micropolitical interaction existed in the decision-making processes of school principals due to formal and informal groups' participation, a consequent of School-Based Management Committee (SBMC) system.