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This study was an attempt to investigate the effect of fostering cooperative learning on EFL learners’ overall achievement. To fulfill the purpose of this study, 56 female students of Saba Language School in Tehran were selected from a total number of 90 based on their performance on the Preliminary English Test (PET) and randomly put into two experimental and control groups. The same content was taught to both groups throughout the 24-session treatment. The only difference was that the experimental group was taught through communicative language teaching with the use of cooperative learning activities, which consisted of the three-step-interview, think-pair-share, paired annotations, round robin, and learning together, while the students in the control group were taught through the communicative language teaching approach without the cooperative teaching procedure. An achievement posttest within the content taught was given to the students in both groups at the end of the instruction and the mean scores of both groups on the test were compared through an independent samples t-test. The result showed the rejection of the null hypothesis thus concluding that cooperative learning had a significant effect on the overall achievement of Iranian EFL learners.