In recent years, Information and Communication Technology (ICT) has been growing quickly and seems to become an integral of individual life. Since the dawn of the twenty first century, technology has crept into learning and teaching environment. With the growing rate, the education framework has to be improved and ICT be coordinated into school activities. This may be the the thought of the State of Osun in introducing computer into senior high schools to assist delivery of instructions. However, the effect of this introduction on students’ performance has to be considered. Therefore this study investigated the effect of Computer Assisted Instruction in science subjects (Biology, Chemistry and Physics) on senior school students’ academic achievement. One school in each senatorial district initially supplied with computer tablets by Osun state government before other schools were purposively selected for the study and an intact science class was randomly selected from each. 152 students participated in the study. One group pretest-posttest experimental design was used for the study. Students’ examination scores served as instruments. The examination questions . Results showed difference between the mode of instruction and students’ achievement in each of the aubjects. The difference was highest in biology; physics and chemistry trail behind respectively. Recommendation is that instructions should be assisted with computer in teaching science subjects. However, for better outcome in chemistry, more effort should be incorporated in the use of Computer Assisted Instruction (CAI).