Abstract: The aim of this research is to determine the retention effect of computer assisted instruction (CAI) on student’s achievement for teaching the chemistry topics. The research included the structure of Atom, Air and carbon units of 8th grade chemistry lesson. In this research 80 students were structured as both control and experiment groups. Traditional instruction (TI) method is used for control group while traditional instruction with teacher supervised CAI method is used for experiment group. Chemistry subject test was applied as pre test and post test to both groups. To check the retention a delayed post test (Retention test) was administered 1 month after the post test. The results of students were analyzed using t-test statistic. The results shows that in the computer assisted instructions the students retained the concepts for a long period of time as compared to the traditional lecture method.