This paper examined principals' quality assurance strategies in secondary schools in Osun State, Nigeria. The study adopted a descriptive survey research design. Stratified random sampling technique was used to select 10 male and 10 female principals, and 190 male and190 female teachers. "Secondary School Principal Quality Assurance Questionnaire" (SSPQAQ) and "Students' Academic Performance Checklist" (SAPC) were used to collect data for the study. The instruments were validated, and test-retest method used to determine their reliabilities yielded 0.74 and 0.71 respectively. One main hypothesis and five operational hypotheses were formulated to guide the study. Descriptive and inferential statistics were used for data analysis. The study further showed that principals' gender and school location had no significant impact on students' academic performance. The implication of the findings is that ability of the principals to put in place strategies that would ensure quality in the production of school output is a critical factor in determining school effectiveness. It is against this background that the paper concludes that quality assurance is a function for all stakeholders within the school system. Because it is a management function, the principal should coordinate teachers and other staff members to see that their activities are geared towards ensuring quality in the schools. As part of quality assurance strategies, the principal should recognize the philosophy of education in all aspects of school activities.