This study aimed at investigating the impact of using computer assisted grammar teaching on the performance of Jordanian pupils studying English as a foreign language. The sample of the study consisted of (212) secondary pupils distributed randomly on four experimental groups and four control groups. To find out the impact of a software program on students performance in grammar, the researcher designed an achievement test and used it as a pre-test and a post test for the experimental and the control groups. An Analysis of Covariance (ANCOVA) was used to find out the effect of the instructional program on the pupils' achievement in the passive voice. The findings of the study revealed that: 1. There were statistically significant differences (a < 0.05) between the pupils' achievement mean scores in grammar attributed to the instructional method of teaching. This difference is in favor of the students in the experimental group. 2. There were statistically significant differences (a < 0.05) between the pupils' achievement mean scores in grammar attributed to stream of study. This difference is in favor of the scientific stream students. In light of the findings of the study, it was recommended that English language teachers use computer assisted instruction in their teaching.