Background: Transformational leaders help subordinates discover their role in helping the organization achieve its mission, increasing their commitment and organizational citizenship. Aim of the study: Examine the mediating effect of transformational leadership between organizational commitment and organizational citizenship behavior among nursing staff. Subject and Method: A descriptive correlational study, conducted at Alrajhy hospital, Assiut university children's hospital and Assiut University heart Hospital by using convenience sample of 100 nurses from each hospital. Data collection tools: tool (I): Personal characteristics, Tool (II): Transformational Leadership Questionnaire, tool (III): Organizational Citizenship Scale, tool (VI): Organizational Commitment Scale, Results: the highest mean score as regards to TLS is found in Heart hospital. While the highest mean scores of OC and OCB are found in Alrajhy Hospital. There is a highly significant positive correlation between TLS and OCB, Also there is a highly significant positive correlation between OCB and OC among nursing staff. Conclusions: OC has a direct positive and significant impact on OCB, TLS has positive and significant effect on OC and OCB, and finally TLS has a mediating effect on the relation between OC and OCB among nursing staff. Recommendations: Conduct an in-service training programs to reorient the nursing leaders with the new styles of leadership, using the leadership strategies that improve nursing staff commitment and organizational citizenship behaviors.