The main objective of this study was to determine the effect of Computer-Assisted Instruction on the performance in Mathematics III of 60 purposively selected third year high school students of St. Anthony’s College, school year 2007 – 2008. Employing the quasi-experimental design, the 6-week study focused only on topics in triangles, polygons, parallel lines and transversals, angles of triangle and polygon, figures formed by parallel lines, and congruence of triangles. The instruments used to gather data were the validated teacher-made achievement test in Mathematics III (given as pretest and posttest) and Computer-Assisted Instruction Program. The pretest was given to the subjects to determine their initial knowledge in Mathematics III. The experimental group was taught using Computer-Assisted Instruction while the control group was taught through the Lecture Method. At the post-experimental stage, a posttest was given to the subjects in both groups. To describe the students’ level of performance, the mean and standard deviation were used. The significance of the difference in the mean scores of the pretests between the two groups, significance of the difference in the mean scores of the posttests between the two groups, and the significance of the difference in their mean gain performances were determined through t-test for the uncorrelated means; while the significance of the difference between the pretest and posttest performances of each group was determined using the t-test for correlated means set at 0.05 level of significance for a two-tailed test. All statistical computations were computer-processed through the Statistical Package for Social Sciences (SPSS) software. The results revealed that the experimental and the control groups were comparable at the start of the experiment. In terms of pretreatment and post-treatment within group, no significant difference was found. Similarly, no significant difference was found in the mean gains between the control and the experimental groups. However, a significant mean gain was observed in the pretest and posttest scores of both groups.