The purposes of this research were: 1) to develop the computer-assisted instruction (CAI) lesson about addition, subtraction, and multiplication with decimal notation, 2) to study academic achievement of student those study from CAI lesson before and after learning and 3) to examine the student satisfaction toward the developed CAI. The samples of a population were 72 fifth-grade students of Bannongkhla School, Chanthaburi Province, Thailand. The instrument consisted of 1) CAI on additional, subtraction and multiplication with decimal notation for fifth-grade student 2) an achievement learning test and 3) the satisfaction rating scale form. The findings showed that the efficiency of the CAI lesson is 80.97/81.02.  The student achievement gained after learning a lesson by CAI lesson were found significantly higher than the score obtained before at the level of .05 and the students who received the treatment also showed an excellent satisfy towards the developed CAI.