The present study is an experimental one in nature, to find out the effectiveness of CAI package on in Physics of IX std. students. For this purpose a CAI package was developed and validated. The validated CAI package formed an independent variable of this study. The dependent variable is students' achievements in physics content. In order to find out the achievements in Physics among IX standard students, the investigator developed and standardized an achievement test on Physics unit entitled "Work, Power and Energy" prescribed for IX std. by Tamilnadu Textbook Society. It is a pre-test-post-test equivalent group design. In order to have randomization in the sample the investigator selected two schools in Madurai city. The Experimental and Control groups were identified in both these schools. Before the treatment both experimental and control groups were equated based on their performances in entry level test. The selected experimental and control groups were administered Pre test, developed and validated by the investigator. The treatment was conducted in four equal sessions. After all the treatments both the groups were administered Post test. Data were collected from the students. They were tabulated and statistical treatments were applied. The analysis results are discussed in this paper.