Wikipedia is the world's largest multilingual free-content encyclopaedia written by users collaboratively. It is interesting to investigate why individuals have willingness to spend their time and knowledge to engage in it. In this study, we try to explore the influence of self-concept-based motivation and individual attitudes towards Wikipedia on individual's knowledge-sharing intention in Wikipedia. Members from Wikipedia were invited to participate in the investigation. An online questionnaire and structural equation modelling technology was utilised to test the proposed model and hypotheses. Analytical results indicate that internal self-concept-based motivation significantly influences individual's knowledge-sharing intention. Further, both information and system quality have significant effects on individual's attitude towards Wikipedia, and therefore, influence the intention to share knowledge in it.