The study investigated the effect of computer- assisted instruction on academic achievement of Nigeria Certificate in Education Physics students of different abilities, in Niger State, Nigeria. Six research questions were raised and six null hypotheses were formulated to guide the study. The study adopted pretest and posttest quasi experimental control group design. Two hundred and eleven (211) NCE II students from the two colleges of education in Niger State were used as research sample. Purposive sampling techniques was used to choose the two schools while balloting method was employed to assign the colleges into experimental and control groups. The researcher developed computer- assisted instructional package on Mechanics and Properties of Matter which was used as treatment instrument for experimental group while control group were exposed to traditional (lecture) teaching method. The instrument for data collection was Mechanics and Properties of Matter Achievement Test (MPMAT) which was validated by experts in science education. A 40- items multiple choice objective type achievement test covering six topics in Mechanics and Properties of Matter was used to collect data for both pretest and posttest. A reliability coefficient of 0.78 was obtained using the data collected from pilot test and analyzed using Pearson‟s Product Moment Correlation Coefficient (r). The data of the first five hypotheses were analyzed using t- test and the sixth hypothesis was analyzed using one way Analysis Of Variance (ANOVA) statistics. The findings of this study revealed that experimental group performed better than the control group, no statistically significant difference in the performances of both male and female students taught using CAI which implies that it is gender friendly and there is no significant difference in the academic achievement among low, medium and high abilities taught using CAI. It was recommended among others, that Physics Lecturers should use computer software in teaching physics in order to enhance student‟s achievement in physics and Physics Lecturers should acquire computer literacy, so that he/she can develop any computer assisted instruction package.