As a new phenomenon of the software industry, Open Source Software (OSS) development has attracted many research interests. Examining what motivate participants to be involved in OSS projects is one of the recently heated research areas. This study is motivated by the significance but lacking evidence on how personality traits may affect participants' task effort on OSS projects. In particular, we investigate how personality traits namely psychological needs for autonomy and competence, and one's altruism interact with motivations. Following Self-Determination Theory, we differentiate types of motivation in OSS communities. In addition, drawing upon the Affective Event theory, we submit that personality traits moderate the relationships between task effort and both external and identified motivations. The research model is largely supported by data from 204 participants in various OSS projects. Theoretical contributions and practical implications are discussed.