Many research studies have investigated the effects of peer feedback in writing classrooms because it can be applied in L2 writing teaching to great advantage. However, not many studies have conducted in-depth investigations of the incorporation of peer feedback into revision. This study is an attempt to fill this gap. This 11-week case study reports the experiences of written peer feedback of 92 English-major students. The study examines the quality of trained written peer feedback and the effects of trained written peer feedback on students’ revisions. Based on the analyses of the written feedback participants received and comparisons of their initial and revised drafts, the study shows that most of the peer comments were revision-oriented and the quantity of accurate comments was remarkably higher than the quantity of mis-corrections. In addition, most of the revisions in the second drafts were triggered by peer comments, and the writing quality was significantly improved among both low and high level writers.