In knowledge economy, organizational knowledge is considered to be a critical strategic resource which may help the organization to achieve competitive advantage; therefore, knowledge sharing, as one of the knowledge management processes, attracts the attention of both researchers and practitioners. Knowledge sharing may bring many benefits such as personal development for the employees and knowledge accumulation for the organization. However, at the same time it is very challenging because employees may be very mobile, too occupied to engage in knowledge sharing, or just unwilling to share their valuable knowledge. Nevertheless, although many organizations have started to invest heavily in various knowledge sharing mechanisms, quite often these processes are not effective because various organizational and individual factors impede the usage of those mechanisms. This study seeks to increase the understanding of how and which of such factors affect knowledge sharing in voluntary project-based organizations. Ten in-depth interviews have been conducted with the project managers in AIESEC, the largest international student organization, in order to find out what KS mechanisms they use, and what enables or hinders KS in this organization. It was found that in this organization KS takes place at all organizational levels, and the most popular KS mechanisms are documents and social interaction. Various documents include planning and tracking tools, proposals for sponsors, budget spread-sheets, feedback forms from participants and companies, and reports about functional areas. Social interaction comprises individual and group meetings, including trainings, coaching or mentoring, conversations over the phone and software Skype, discussions in conferences and communication in social groups online. So there is a balance between the KS mechanisms used to personalize and codify knowledge. However, the individualized KS mechanisms dominate on the individualization- institutionalization dimension. Also AIESEC members share all types of knowledge: tacit and explicit, individual and collective. Factors affecting KS can be categorized in 5 groups: Organizational context, Interpersonal and Team characteristics, Cultural characteristics, Individual characteristics, and Motivational factors. As KS in AIESEC takes place quite intensively, not surprisingly more KS facilitators were identified. The most significant ones in each group are as follows: the organizational culture and structure; diversity and strong social ties; willingness to help, and structure and exactness; self-efficiency and personal characteristics such as talkativeness, open-mindedness, empathy, motivation, responsibility and ambitiousness; perceived personal benefits, interpersonal trust, and organizational commitment. The few factors identified that might impede KS in AIESEC are the lack of time, lack of KS regulation and some negative cultural attitudes.