The purpose of this study was to determine the effect of computer aided instruction and traditional method of instruction on the junior secondary school students achievement in mathematics. Four research questions and hypotheses were stated and tested in the study. The design of the study was the pre-test post-test control group experimental design. The sample for the study consists of 270 randomly selected students from three junior secondary schools in Edo state. The instruments constructed by the researcher were content validated by three experts in maths education, computer science and evaluation. The reliability coefficient of the instruments were 0.75, 0.79 for Test of Understanding of Mathematics using Computer (TUMUC) Test of Understanding of Mathematics using the Traditional Instruction (TUMTI) respectively. The instruments were administered on the students after treatment and data collected were analysed using students t-test, and Multiple Analyses of means using Duncan's test. The findings of the study were that CAI significantly influenced students' achievements in mathematics than the programmed instruction than the traditional instructional method. The findings also revealed that male students profit better than the female students. (Contains 3 tables.)