An online discussion forum becomes more significant in facilitating people to share opinions. Companies must understand how consumers use the online forum. We study an influential online discussion forum in Thailand, Pantip.com, in order to classify user behaviours in the cosmetic discussion forum. We adopt the concept of genre and genre repertoire to assist our content analysis. We analyse 77 discussion threads, which contain a “cosmetic” keyword during a 6-week period in late 2014. Our preliminary analysis shows that the genre repertoire found on Pantip.com consists of three genres: (1) Information sharing, (2) Opinion discussion, and (3) Question asking. A theoretical contribution from this study is that we show how genre theory can be used to describe an online discussion forum, and a practical contribution is that business companies could adopt the insights generated to form their communication practices to customers, which are inline with existing genres