The study investigated the predictive power of school based assessment scores on students' achievement in Junior Secondary Certificate Examination (JSCE) in English and Mathematics. Two hypotheses tested at 0.05 level of significance guided the study. The study adopted an ex-post facto research design. A sample of 250 students were randomly drawn from ten (10) schools out of twenty (20) Junior secondary schools in Obio-Akpor Local Government Area of Rivers State. The instrument used for data collection was titled "Students Academic Record Inventory" (SARI). The instrument involves the students SBA score for JS1, JS2, JS3 and JSCE scores in English and Mathematics. The data generated were analyzed using multiple regression. The result revealed that the combination of the SBA scores significantly predicted students' English and Mathematics achievement in JSCE, and there are relative effectiveness of SBA scores in students' English and Mathematics achievement. Based on the findings of the study, recommendations were made including that school-based assessment should be encouraged and made compulsory for every secondary schools especially in core subjects which include English and Mathematics.