The mastery of the writing skills is crucial, not only among the school children, but also to everyone. The use of Cooperative Learning (hereafter, CL) has become increasingly popular in recent years as pedagogy trends worldwide. It has shifted from teacher-centred to learner-centred methods. Among the methods endorsed in teaching writing is the application of CL. This research investigated the effects of CL to improve the writing skill of ninth grade students in a middle school in Kuala Lumpur. This research used the quasi experimental design, with pre-test and posttest of the narrative essays as the instruments. The data were further analysed by employing descriptive and inferential statistics. The students' writing were scored on the five writing components, they are vocabulary, organization, grammatical accuracy and mechanics. The results showed that the students had increased in their writing scores from the pre-test to the post test after the application of CL in the class. Subsequently, the results indicate positive effects of CL in improving the writing skill of students at the school, and they are also discussed in the paper.