In order to compare the effectiveness through Computer Assisted Instruction (CAI) and Traditional Lecture Method for teaching Physical Science, an experimental research was conducted on the students of class X studying under West Bengal Board of Secondary Education in Kolkata District in West Bengal. A two group (Experimental and Control group each containing 9 students) randomized matched posttest Equivalent Design was taken. A unit on Lens was taught with the two groups separately and the same posttest was applied on both the groups to test the hypotheses. Lesson plan and achievement were designed by Investigator himself. The mean scores of the students taught through CAI significantly better than the Traditional Lecture Method as found by one tailed test. This study revealed that CAI is more effective than the Traditional one.