This study attempted to find the effect of Computer Based Instruction (CBI) on achievement in mathematics of secondary school students. An experimental study was conducted using two equivalent group design among IX standard students in R.A.N. Matriculation school, Kavarapetti, Thiruvallur district, Tamilnadu, India. The sample was divided into two groups, each consisting of 35 students. The effectiveness of CBI was measured using gain scores (difference between pre-test Post-test scores) of both the groups. The results revealed that the gain score of experimental group was significantly better than the control group students in Mathematics especially in Algebra. Hence it is concluded that computer based instruction plays vital role on achievement in mathematics among standard ix students.