The purpose of this study was to investigate effect of using cooperative learning technique on EFL students’ writing performance at university level. The study employed a quasi-experimental research design. Two intact classes of second year undergraduate students who were taking Basic Writing Sills course at Jimma University participated in the study. The two classes were assigned to treatment\ experiment and control groups. The treatment group consists of 48 students while the control group comprises 41 students. The treatment group was taught writing in English using cooperative learning technique and the control group was taught writing in English using individual learning technique. Data were collected through paragraph writing tests (pretest & posttest). The collected data were analyzed using descriptive and inferential statistics. Independent samples t-tests was conducted to determine whether or not there was a statistically significant difference between the mean scores of the writing performances of the treatment group and that of the control groups. Paired sample t-tests were also conducted to determine whether or not there was statistically significant difference between the pretest and the posttest mean scores of each group. The results of the independent samples t-tests indicated that the writing performances of the treatment group and that of the control group were similar before the treatment; however, after the treatment, the writing performance of the treatment group was significantly better than that of the control group. The results of paired sample t-test also showed that the writing performance of the treatment group was significantly improved after the treatment due to the implementation of cooperative learning technique in writing classes. Based on the results it was concluded that using cooperative learning technique improves EFL students’ writing performance at university level in the context of Ethiopia. Therefore, it is recommended that English teachers should use cooperative learning technique in writing classes in order to help university undergraduate students to improve their writing performance. Material designers in the country should also incorporate writing activities that allow students to use cooperative learning technique in the process of learning writing.