Recently, co-teaching has been employed as an instructional technique to accelerate and facilitate second or foreign language learning process. This study was set up to investigate the effectiveness of co-teaching on EFL students' writing ability and also to probe into their attitudes towards co-teaching. Two male Iranian EFL teachers with the same language experience and academic degree represented the writing co-teachers. Furthermore, two available classes of 20 students were utilized. Oxford Placement Test was first used to ensure the homogeneity of the participants in terms of language proficiency. Second, a writing pretest was run to assess the participants’ writing ability prior to the implantation of co-teaching. Third, the experimental group went through an eight-week instructional period with the two writing instructors while the control group was taught with a single teacher. Fourth, a writing posttest was administered to see how different the two groups were regarding their written performances. Finally, a semi-structured interview was also conducted to delve into the students’ overall attitudes towards the effectiveness of co-teaching. Regarding the qualitative phase, the researchers conducted the interviews and then transcribed them. Independent samples t-test results indicated that the experimental group outperformed their counterparts in the control group with regard to their written performances and participants in the experimental group held positive attitudes toward co-teaching. It behooves the stakeholders to delve into the potential pluses and minuses of co-instruction and determine how effective it can be for their intended audience under various pedagogic settings.
