A peer-based reviewing on writing often leads to improving learners’ writing quality. Particularly, reviewing by writing comments lifted writing quality more greatly than reviewing without writing comments. This implies that learners’ hard working on peers’ writing engenders benefits on their own writing. This study attempted to explore which cognitive mechanism underlies in this cooperative learning system. We questioned whether learners’ use of metacognitive strategies during reviewing might facilitate learners’ self-learning. In a preliminary study, participants were asked to categorize their comments on peers’ reports. Results from the study revealed that learners who categorized comments used metacognitive strategies more increasingly than those who did not. Controlling for the amount of using metacognitive strategies, writing quality was improved numerically more when learners were engaged in the categorization task than when they were not. With further data, we will examine to what degree learners’ metacognitive learning during reviewing contribute to writing improvement.