This study aimed at examining the effect of cooperative learning on developing grade 11 EFL learners writing performance. A quantitative method with quasi experimental pre and post - test design with a control group was employed for the study. The participants were two sections of 11 th graders (n=82) selected using purposive sampling technique. They were assigned in to experimental (n=42) and controlled (n=40) groups through a lottery system. The data were collected through writing tests ( pre-test and post-test) and questionnaire. The experimental group was taught using a cooperative learning strategy namely Round Robin for eleven weeks whereas the controlled group was taught using the usual instruction for teaching writing. The overall writing performance and their writing fluency, accuracy and complexity of the two groups were measured before and after the experiment. Independent samples test and paired sample t-test were employed to analyze the data as statistical tools to find out if there was significant group as well as pre-posttest mean score differences in overall writing performance and the three aspects of writing. The data analysis before intervention indicated that both groups were almost similar in their overall writing performance (t (80), .02, p >.98) and in the three aspects of writing. However, after the treatment, the data analysis by independent sample t-test was found t(80),-3.18, p<.002 indicating that the experimental group significantly overtook the control group on the overall writing performance post–test with the domination of cooperative learning over the usual method. Likewise, they were significantly different in the fluency and accuracy measure, however insignificantly different they were in the complexity measures. Besides, data analysis using paired sample t-test for the experimental group appeared highly significant at p<.001 for the overall writing performance as well as for the fluency and accuracy measures although the complexity measures pre-posttest score comparison were insignificant. And the writing performance pre-posttest mean scores comparison for the control group was insignificantly different. Furthermore, data analysis from the questionnaire revealed that the experimental students perceived the cooperative learning writing positively. Therefore, the findings of the study advocates that cooperative learning helped preparatory Students significantly to enhance their EFL writing performance particularly in term of fluency and accuracy, though not in the complexity aspect; in light with these findings, therefore, EFL teachers are recommended to use CL in their writing classes. Besides, recommendations for pedagogical practice and for further studies on effects of this learning strategy on writing were recommended.