The study investigated Problem-based Instructional Strategy and Numerical ability as determinants of Senior Secondary Achievement in Mathematics. This study used 4 x 2 x 2 non-randomised control group Pretest-Posttest Quasi-experimental Factorial design. It consisted of two independent variables (treatment and Numerical ability) and one moderating variable (gender). The subjects consisted of 196 Senior Secondary School Students (SSS II) 90 males and 106 females. A stratified cluster sampling technique used to select the students from four equivalent co-educational Senior Secondary Schools that are distinctly located from each other within the town of Ijebu-Ode, Ogun State. Two valid and reliable instruments (Numerical ability test and Mathematics achievement test) developed to collect data for the study. Their difficulty indices of the items selected ranged from 0.50-0.65, discriminating indices of 0.45-0.71 and reliability index values were obtained to be 0.76 and 0.83 respectively using Kuder-Richardson (KR-20). The results show that the three variables when combined (treatment, numerical ability and gender) jointly accounted for 28.0% of the variation obtained in the students school in mathematics; there is no significant interaction effect on the numerical ability and Gender on students' achievement in mathematics (F (1,196) = 0.242, P > 0.05). And treatments have no significant influence on the students' achievement in mathematics. It is therefore recommended that appropriate courses need be introduced into teacher education programme for the training teachers in these students of designing and developing useful learning packages for mathematics instructions.