This study was carried out to examine the Relationship between Principals’ Managerial Skills and Administrative Effectiveness in Secondary Schools in Oyo State, Nigeria. The researcher adopted a correlation survey research design. Simple random sampling technique was used to select twenty (20) secondary schools. In each of the school, 10 teachers were selected. On a whole, 200 teachers served as subjects for this study. A researcher- designed questionnaire titled: “Principal Managerial Skills Questionnaire” (PMSQ) was used to elicit relevant data for the study. To ascertain the validity of the instrument, content validity was adopted. Also, the reliability co-efficient of the instrument was obtained through test-re-test method and the co-efficient of 0.65 was obtained. Four research hypotheses were formulated to guide the study. Thus, Pearson Product Moment Correlation Statistics was used to test the research hypotheses at0.05 significance level. The findings revealed that there was significant relationship between principals’ managerial skills and administrative effectiveness (Cal. Rvalue= 0.246> critical r-value = 0.148).