This study was an attempt to investigate the effects of cooperative learning on Grade 11 students' writing performance in Afar Regional State, Samara Secondary and Preparatory School, in focus. More specifically, the study addressed the following research questions: (1) What are the effects of cooperative learning on students' writing performance?(2) Which one of the components of writing most benefited from the cooperative learning approach? And the following hypotheses: (Ho) There is no significant difference in the mean gain scores of the writing performance before and after the incorporation of cooperative learning and (H1) There is a significant difference in the mean gain scores of the writing performance before and after the incorporation of cooperative learning. This research project was mainly experimental in design. Pre-test and post-test measures were analyzed using a t-test statistical procedure. The main subjects of the study were two sections of Grade 11 students. Moreover, 60 students, 30 each from two natural science classes were randomly grouped as experimental and control groups. The effects of CL on students' writing ability was examined through the pre-test and the posttest. The results of the pre-test showed that there was no significant difference in the students' writing between the control and experimental groups prior to the experiment. The results of the post-test showed that there was a significant difference between the control and experimental groups in students' writing abilities (P<0.05). Therefore, the null hypothesis was rejected and the alternative hypothesis was proved to bring change on students' writing ability. Similarly, the study also identified that organization is the basic component of writing skill that benefited most from the cooperative learning approach. However the current study was showed positive effects of CL on students' writing performance, in the future researches can be conducted on the effects of CL on other language skills.