This study examined learning Social Studies by investigating the effectiveness of mastery approach on students' performance in Social Studies of two groups of students drawn from Osun State College of Education, Ila-Orangun and University of Uyo, Ila-Orangun Study Centre which consisted of 200 level Social Studies students and 200 level degree Social Studies students. A unit of course peculiar to both NCE and Degree Students in Social Studies was taught. SOS 221 titled; Issues and Problems of National Development for NCE Students and SSE 225 titled; Issues and Problems of Modern Society was broken down and presented sequentially to learners at different levels; the difficulty of the learning task varied according to the level of learners involved. In all 60 students, 30 from each level were involved. The research design was the posttest only control group design, which allows research to be conducted without any pretest. Results showed a clear-cut conclusion on the effectiveness of mastery learning approach on students' performance as students in the two categories performed better in Social Studies compared with conventional approach to learning. This result was found to be significant at the 0.05 level. Some deductions were made on the results and the need for further research work is advocated. (Contains 2 tables.)