The study examined the effects of Advance Organizer teaching strategy on students’ academic achievement in Basic Science in secondary schools in Ado Local Government Area, Ekiti State, Nigeria. The study was a pretest, posttest, control group quasi-experimental design. Purposive and stratified random sampling techniques was used to select a total sample of 160 public JSS I Basic science students (this sample was divided into the experimental and control groups in ratio 1:1; meaning that, 80 students from each group) from four Junior secondary schools in Ado Local Government Area, Ekiti State. Two schools each for Experimental and Control groups. Two null hypotheses were formulated and tested at 0.05 level of significance. The instrument for this study was Basic Science Achievement Test (BSAT) and the treatment package used for the study was tagged: Advance Organizer Instructional Package (AOIP). The data collected were analysed using t-test and ANCOVA statistical analysis packages. The results of the analyses showed that no significant difference existed between the achievements of students in experimental and control groups involved in the study at pretest (this indicated initial academic homogeneity of the groups). However, students’ achievement in the experimental group at post-test level was found to be significantly better than that of the control group. This showed that Advance Organizer teaching strategy significantly influenced students’ achievement in Basic Science in Junior Secondary School. The implications of the results on students’ achievement in Basic science are discussed. Based on the findings of the study, conclusion and appropriate recommendations were made.