Monitoring one's own performance accurately is essential for information-processing and self-regulation, which are indispensable in an online learning environment. In this paper, the effect of metacognitive monitoring on interaction in an asynchronous online discussion forum was investigated. Transcripts of this forum, which was integrated in a face-to-face traditional undergraduate course, were analyzed in terms of Henri's (1992) cognitive and interactive dimensions. Metacognitive monitoring was measured by the “cognitive-title” technique, which makes use of the cognitive dimension. Analysis indicated that metacognitive monitoring had a large effect size on interaction in the asynchronous online discussion forum. The study particularly suggests that labeling messages with a “cognitive-title” should be a rule of asynchronous online discussion forum protocol. Moreover, this activity might also be a useful tool for improving metacognitive monitoring in a virtual environment.