This paper analyzes the comparative effectiveness of using Computer-Assisted Instruction (CAI) method and traditional teaching method in mathematics on higher secondary school students. A purposive random sample consists of 180 students chosen from three government higher secondary schools in Coimbatore district of Tamil Nadu. The students of the experimental group learnt the XI standard mathematics topic namely "Differential Calculus" using CAI, whereas the students in the control group learnt the same topic through the traditional method of teaching. The research design chosen has been the pretest-posttest equivalent groups design. All instruments (the pretest, the posttest and the retention test) contained the questions belonging to four different cognitive domains: knowledge, understanding, application and skills. The reliability of the instrument (attitude scale) has been measured by using internal consistency reliability, which is measured by Cronbach Alpha. The data has been analyzed using mean, S.D., Chi-square, ANOVA and t-test. Findings of the study clearly point out that significant increase in the mean gain scores has been found in the posttest scores of the experimental group. Significant differences have been found between the control group and experimental group on posttest gain scores. The experimental group, which has been taught using the CAI showed better performance in learning and also the CAI is an effective medium of instruction for teaching mathematics at higher secondary level.