A long standing dilemma of online communities is that a small group of community members account for a disproportionate amount of contributions. Prior studies built on intent-based explanations cannot fully explain the phenomenon. This paper introduces the concept of habit formation as a key driver of individual contributions and investigates how habit is formed and how it influences individuals’ participation behavior in online communities. We propose that a threshold of behavioral repetitions exists for individuals to develop a habit. Once the threshold is surpassed, the habit of participation grows stronger and becomes self-reinforcing. We also propose that habit formation weakens the influence of reciprocity, social capital and competition on user participation in virtual communities. Using a panel data of 130,882 postings across 115 discussion boards, we find support for all the hypotheses. Our analysis contributes to the emerging literature on routinized information technology use.