Students need to be skilled writers to get involved in a literate society. Providing learners with effective and appropriate teaching approach can lead to an easier and better mastery of skills. One of these approaches may be co-teaching. This study investigates the effect of co-teaching for Iranian advanced EFL learners' ability to write. The co-teaching approach was taken as a treatment through which two teachers were cooperating simultaneously during the teaching writing process in the same class with the experimental group. The participants of the control group were taught traditionally by a single teacher during all class sessions. According to the data analyses after comparing the two groups, co-teachers have been more successful in improving the overall writing ability of the treatment group. The underlying rationale is one in which teaching a class with two teachers may first add to the variety of teaching styles, feedback giving, consultation and reviewing the teaching process that can lead to better teaching practice and also doubling the class time by increasing teacher-student interaction time.