The main purpose of this study was to see the effects of collaborative writing instruction on students’ writing performance and social skills in English language class at Wurgessa General Secondary School. This research design was a quasi-experiment, used to allow the researcher to gather information and interpret data for the purpose of clarification. To check the validity and reliability of the instruments, pilot study was carried out on a group of people similar to the target participant. Pre-test and post-test were given for both control and experimental group samples to measure their writing performance. In addition, pre and post questionnaire were also given for those control and experimental group samples to measure their social skills, and both test and questionnaire were analyzed quantatively. The researcher used the target population of 300 students of Wurgessa General Secondary School, two sections with a sample size of 74 students determined by simple random sampling. To this effect, an intervention program was conducted for two months to enhance the students writing performance and social skills of the experimental group.Teaching material which mainly consisted of collaborative writing activities was prepared and used for experimental group students. Grade ten English student’s text book and research on EFL collaborative writing tests were the main sources for the preparation of the teaching material. The results of the pre-test showed that there was no significant difference in students writing performance between the control and experimental groups. However, the results of the post tests showed after intervention, experimental group students scored better than control group students. In addition, the data obtained from the students’ questionnaire also indicated that after intervention, students in the experimental group had a positive social skills improvement towards collaborative writing instruction. On the basis of the finding of the study, it was concluded that collaborative writing instruction improved students writing performance and social skills. Based on the finding of this study, conclusions and recommendations were made.