BACKGROUND: Nurses are expected to empower their clients, but they cannot do so if they themselves feel powerless. They must become empowered before they can empower others. Some researchers have emphasized that understanding the concept of power is an important prerequisite of any empowerment program. While many authors have tried to define the concept of power, there is no comprehensive definition. This paper is an attempt to clarify the concept of power in nursing. It also would present a model describing the factors affecting nurse empowerment. METHODS: We chose the grounded-theory approach for analysis of the participants' experiences and their viewpoints regarding the concept of professional power in nursing. Semi-structured interviews and participant observation methods were used to gather the data. Forty-four participants were interviewed and 12 sessions of observation were carried out. The constant comparative analysis method was used. RESULTS: Six main themes emerged from the data: "Application of knowledge and skills", "Having authority", "Being self-confident", "Unification and solidarity", "Being supported" and "Organizational culture and structure". According to the participants, nurses' power is influenced by these six variables. A theoretical model was designed to represent the interrelationships between these six variables. CONCLUSIONS: Nurses' power depends on gaining and applying professional knowledge and skills. Delegating authority and enhancing self-confidence of the nurses also help them to apply their knowledge in practice. Unification of the nurses and their mutual support play the key roles in development of their collective power and provide a base for better working conditions, professional independence and self-regulation.