Abstract The purpose of this research is to develop and evaluate the efficiency of Computer Assisted Instruction, to study pre-post achievement by Computer Assisted Instruction and to study students' opinion about Computer Assisted Instruction : Introduction to Statistics lesson on "Introduction Theory of Probability". A sample in this research were 51 sophomore students of Bangkok University in 2002 academic year selected by cluster sampling. The instruments used in this research were Computer Assisted Instruction, which had the standard criterion 80/80, Achievement Test and Opinion Questionnaire. Data was analyzed by Percentage, Mean, Standard Deviation and pair t-test. The results of this research shows : 1) The efficiency of Computer Assisted Instruction (CAI) was 82/81.33 which was higher than the standard criterion set (80/80). 2) After learning from Computer Assisted Instruction (CAI), students got higher scores. The difference is significant at the level .05 and 3) The students' opinion about Computer Assisted Instruction (CAI) indicate that most of them like Computer Assisted Instruction (CAI) because it is interesting and not boring. Also, they understood lesson better and want to continue study using Computer Assisted Instruction (CAI) again in this course and others.