This research aims to provide an understanding of knowledge sharing behaviour through the adaptation of two major theories, the Theory of Reasoned Action (TRA) and the Theory of Planned Behaviour (TpB) which were developed by Martin Fishbein and Icek Ajzen from social psychology. Exploring knowledge sharing from a social psychological perspective provides an understanding of the process an individual goes through to make the decision to share his/her knowledge with others as well as the different psychological factors facilitating or hindering knowledge sharing behaviour. Research findings are based on a web-survey of 158 group members of one online community of educators, Tapped In (TI). TI is a professional online community developed and supported by the Stanford Research Institute (SRI International), and directed to enhancing the quality of teaching by developing web-based opportunities of learning and engaging.Proposing an extended theoretical model of knowledge sharing behaviour in online community, this research tested the predictive power of five major variables on the individuals' intention to share expertise and knowledge. Variables included attitude, subjective norms, descriptive norms, controllability and knowledge sharing self-efficacy (KSSE). The study found that normative pressures, including subjective norms and descriptive norms, had a strong influence on the formation of the individual's intention to share in this specific online community. Knowledge sharing self-efficacy also was found to significantly account for explaining the individual's motivation to share his/her knowledge with other members. Attitude and controllability were not found to have significant impacts on the formation of intention.