This study investigated the effects of the cooperative and competitive learning on academic performance of students in mathematics in order to find out which one of them is the more effective learning strategy. The sample of the study was 400 Senior Secondary Schools III, Mathematics students made up of 240 boys and 160 girls randomly selected from four out of five States in South West Nigeria. Quasi experimental design was adopted for the study. Two instruments were used namely Mathematics Pre-Test Achievement Test (PTAT) and Post-Test Achievement Test (PAT) to collect data. The data collected in this study were subjected to Z-test analysis at [alpha] = 0.05 level of significance. The findings revealed that cooperative learning strategy is more effective than competitive learning strategy and that boys performed significantly better than girls in both learning strategies. Based on the findings, cooperative learning strategy should be introduced in our secondary schools in Nigeria. (Contains 4 tables.)