There has been substantial research on the effects of metacognitive instruction on improving L2 learners' language skills. However, little is known about the impact of such instruction on improving the effectiveness of L2 students' collaborative writing. To fill this research gap, we conducted a quasi-experimental study to investigate how such an instructional programme would help Chinese EFL learners' writing development through collaborative writing. A total of 62 post-graduate students from a university in central China were randomly allocated to either an experimental group receiving three weeks of collaborative writing interventions with metacognitive instruction for writing collaboratively, or a control group that was taught via a genre-based approach. Pre-test, immediate post-test, and delayed post-tests were administered with both groups at the onset, immediately after, and six weeks after the interventions to compare their change in writing production, specifically in lexical complexity, accuracy, and fluency. Results revealed that the metacognitive instruction and collaborative writing interventions helped improve participants' writing performance in lexical variation, accuracy, and fluency. However, no significant effects were observed in lexical density and lexical sophistication. We discuss the results for their pedagogical and theoretical implications.