The purpose of this research is to develop also find an efficiency of the computer assisted instruction for an English subject.Furthermore, another objective is to compare a learning achievement between before and after studying the computer assisted instruction. A sample group that is used for this research is thirty primary students in third level from
Primary Demonstration School of Suan Sunandha Rajabhat University, Bangkok, Thailand. In addition, the apparatuses for this research consist of a pretest exam, an exam during a lesson, a posttest exam, and a questionnaire for experts in content as well as technical method.The result of the research discovers that the computer assisted instruction has an efficient value at 82.33/81.60 that is higher than estimation as 80/80. Moreover, an achievement after learning is higher than before learning at 0.5 in a statistical significance. Also, a quality evaluation result of the computer assisted instruction by the experts is in a good rate. In conclusion, this computer assisted instruction for an English subject has a good efficiency that could be used in an English class for the third level of primary students.