Although research about the effectiveness of second or foreign language collaborative writing for jointly written products has proliferated in the last few decades, there has been less examination of whether and how pre-task preparation could maximize the language learning opportunities that collaborative writing can afford learners. Informed by metacognition theory, this study used a mixed-methods approach to investigate the impacts of one technique for preparing students, namely educating them about collaborative writing knowledge. This study compared collaborative writing products from two parallel classes (one with exposure to explicit collaborative writing knowledge and one without) to investigate whether and how knowledge about collaborative writing affected the complexity, fluency, accuracy, and quality of collaboratively drafted essays. The findings indicated that students who were exposed to collaborative writing knowledge outperformed those who were not in terms of the accuracy, fluency, and quality (content, organization, grammar, and vocabulary) of their collaboratively drafted essays. The findings also revealed that knowledge of collaborative writing affects the quality of collaboratively drafted essays from the perspective of metacognitive strategies, namely planning, monitoring, and evaluating one's own writing process.