The unique features and capabilities of online learning are built on the ability to connect to a wider range of learning resources and peer learners that benefit individual learners, such as through discussion forums, collaborative learning, and community building. The success of online learning thus depends on the participation, engagement, and social interaction of peer learners, which leads to knowledge sharing. Thus, without frequent and persistent interaction, it is doubtful whether knowledge sharing can take place in online learning. This study argues that theories about the development and maintenance of social relationships provide a theoretical foundation for understanding the motivation to engage in online knowledge sharing behavior. An Online Knowledge Sharing Model (OKSM) is proposed and empirically tested among undergraduate students using an online learning environment. The model introduces two new constructs--Perceived Online Attachment Motivation (POAM) and Perceived Online Relationship Commitment (PORC), which together explained 71 percent of the variance observed in self-reported online knowledge sharing behavior. The findings provide some explanations for the motivation to share knowledge, and have several implications for the design of the features and capabilities of online learning environments. (Contains 6 tables and 2 figures.)