The purpose of the study was to determine the effects of normal instruction supplemented by the computer-assisted instruction English program Grammar Game on achievement and attitude scores of vocational-technical students in Thailand. The experimental design was a 2 x 2 factorial ANOVA design. One hundred seventy-eight students at the Lanna Polytechnical College in Thailand were randomly selected from the population of 10 classrooms. Four classes were intact groups, with two classes randomly assigned to the experimental groups which received Lecture/CAI and the other two as control groups which received Lecture. The 89 students in each group were divided into high- and low- ability, based on their previous English scores. Subjects received treatment for nine weeks. Pre-test and post-test instruments on achievement and attitude were administered to both groups. The Statistical Analysis System (SAS), and the General Linear Model (GLM) package computer program yielded the MANOVA results. Based on data analysis, the findings were as follows: (1) There was a significant difference between the students in a Lecture/CAI English program and the students in a Lecture English program when they were compared simultaneously on the achievement and attitude scores, F(l, 176) = 18.97, p < .05. (2) There was no significant interaction between the types of teaching methods and levels of ability when achievement was used as the dependent variable, F(l, 174) = .48, p > .05. (3) There was no significant interaction between the types of teaching methods and levels of ability when attitude was used as the dependent variable, F(l, 174) = .06, p > .05. The conclusion was that normal instruction supplemented by CAI improved achievement and attitude scores. On the other hand, the effect of two types of methods on achievement remained the same for high- and low-ability students and so did the effect of two types of methods on attitude. Future research should examine different CAI software packages, CAI within a full year, the effects of ability on achievement and attitude, background knowledge, more levels of independent variables, characteristics of an effective CAI program, and a case study with larger population in Thailand.