The study determined the Impact of School-Based Supervision on Teachers' Performance in Public Secondary Schools in Sapele Education Zone in Delta State, Nigeria. The study has five objectives which includes: determine the impact of Principals' assessment of lesson plans, class visitation, assessment of scheme of work, Teaching Facilities, Learning Facilities on the performance of teachers in Secondary Schools in Sapele Education Zone, Delta State. In line with the stated objectives, five research questions and five hypotheses were formulated for the study. The study used descriptive survey research design, with the total population of 943. Stratified random sampling method was used, 295 respondents were used as sample of the study. Data collected from the respondents were obtained using structured questionnaire. This instrument was validated and pilot tested and a reliability index of 0.85 was obtained.Data gathered from the field were analysed using mean and standard deviation for research questions, while t-test was used for hypotheses at 0.05 level of significance. Findings of the study among others revealed that principal ensure that lesson plan/note developed by teachers create specific knowledge and skills that learners acquire and use at the end of the lesson in most secondary schools in Sapele Education Zone. Also, principal always asked for reports of scheme of work at the end of the week and inspect the scheme of work covered by the teachers in most secondary schools in Sapele Education Zone. The study concluded among others that supervision of lesson plan by principal has helped teachers to be up to date with subject matter. The study recommended that: internal supervision should be frequently conducted in secondary schools in Sapele Education Zone in Delta State and Nigeria in general so as to improve teaching and learning process and regular classroom visitation should be carried out in schools in order to help teachers' keep-abreast with modern trends in teaching.