The present study investigated and compared the effect of cooperative learning method and the whole class traditional method in developing English language of the students of 7th class. The students under control conditions were taught through whole class traditional method and the students in experiment groups were taught through cooperative learning method. The STAD (Student Teams Achievement Divisions) model of cooperative learning was used in this study. Four intact (as it is) groups were taken from boys and girls schools for this study. A pre-test post test control group design was applied. The independent sample t-test procedures were used to compare the control and experimental groups on pre-test and post test scores of achievement test. The results of the study revealed no statistical significance difference on pre-test scores of both the groups which means that the groups were alike in their achievement in English grammar before the experiment whereas the experimental classes outperformed the control classes when compared on post test scores after the experiment. The results based on post test scores showed that the STAD model of cooperative learning had significant effect on the achievement of students, both male and female, in learning English grammar at Elementary level. The effect size was also calculated to determine the magnitude of difference between achievements of experimental and control groups which showed high increase in the achievement of treatment groups.