The purposes of this research were to develop mathematical achievement on ?multiplication? for Prathomsuksa 2 using computer assisted  instruction (CAI) lessons based on  the effectivency standard 80/80 and to compare the students? mathematical achievement before and after learning the CAI lessons.  The sample consisted of 28 Prathomsuksa 2 students studying in the first semester of the academic year 2007 at Thesaban 1 (Wat Chang) School in Bangkhla District, Chachoengsao Province.The instruments were CAI lessons on ?multiplication? an achievement test, and a CAI evaluation form. the E1/E2 formula was used to examine the efficiency of the CAI lessons. 

            The results of the research were as follows : 1) The effectiveness of CAI lessons on ?Multiplication? was 81.35/84.02 which was higher than the standard. The CAI lessons   constructed  by  the  researcher had the efficiency  and  could develop the  students? mathematical achievement.  2) The students? mathematical achievement after learning the CAI lessons was higher than  before  learning the CAI lessons at the .01 level of  significance.

