Background : Nurses are one of the parts of the hospital that are in sever need of organizational citizenship behavior due to the necessity for them to complete these jobs, that is influenced by perceived organizational support and organizational trust. This study: Aimed to investigate the effects of perceived organizational support and trust on organizational citizenship behavior among staff nurses Design: A descriptive design was utilized in this study. Setting: This study was conducted at Zagazig Fever hospital affiliated to ministry of health. Subjects: A convenient sample of 100 staff nurses was used in this study. Tools of data collection: Three tools were used: perceived organizational support scale (POSS), organizational trust inventory (OTI) and organizational citizenship behavior scale (OCBS). Results: Indicated that 88% of staff nurses had low level of perceived organizational support. As well, the majority of staff nurses had low level of organizational trust (89.0%), while, 90% of them had moderate level of organizational citizenship behavior. Conclusion: perceived organizational support was positively and significantly correlated with organizational trust and organizational citizenship behavior. As well, there was positive and significant correlation between staff nurses' organizational trust and their organizational citizenship behavior. Furthermore, organizational trust was highly significant positive predictor of organizational citizenship behavior of the studied staff nurses. Recommendations: Organization should begin organizational support prior to the start of employment, provide the basic needs of nursing staff, establishment of mutual trust among them and themselves and creating a blame-free culture and reward system should be introduced to staff nurses who demonstrate organizational citizenship behaviors. Key words: Perceived organizational support, trust, organizational citizenship behavior, staff nurses.