This paper identifies a set of external factors which may influence on users' perceptions about an information system's ease of use and usefulness underlying user beliefs. The research model was developed based on Davis's Technology Acceptance Model (TAM) along with extending a comprehensive list of organizational, social, individual, and technological factors as the determinants of users' perceptions about a technology. The research model was tested through a quantitative analysis of a dataset collected from 128 users of Project Management Information System (PMIS), software for managing the flow of information across the entire company's projects. Partial Least Squares (PLS) supported the significant impact of the following factors on users' beliefs in terms of Perceived Usefulness: organizational factors including User involvement in system development and Communication channels, social factors including Subjective norms of peers and informal groups and technological factors include Task-Technology Fit; and Quality of relationship with managers and peers; individual factors including Self-efficacy and Personal innovativeness; and technological factors including Compatibility were the most important antecedent of the belief variable in term of Perceived Ease of Use. Finally, the research implications and future directions are discussed.