While several research studies have investigated the impact of peer review training (PRT) on writing quality of L2 students, effects of PRT on revision quantity and quality remain vague. The purpose of this study was to compare revision quantity and quality before and after a scaffolded PRT following Anderson's proceduralization theory. Twenty French-major Vietnamese college students were trained for two months to become acquainted with peer review processes: detection, diagnosis and correction. Texts and feedback from pairs of students were collected and analysed in terms of cognitive revision processes. Mann-Whitney U tests showed significant effects of the scaffolded PRT on detection quantity, diagnosis quantity, diagnosis quality and correction quality. Students made more error detections, more compliments, more diagnoses, and more macro-level detections but fewer micro-level detections in the first draft of their partner's text after PRT. They generated better diagnoses in the first draft of their partner's text and better corrections in the final draft of their text after training. The author recommends that language teachers continue to offer scaffolded PRT to improve students' writing competence.