The human resources domain has interactions and communications with different functional and operational areas in organizations and can therefore have a significant impact on the performance of organizations and the achievement of organizational goals. Therefore, organizations need effective and efficient managers and employees in order to achieve their goals for comprehensive growth and development. In this research, we intend to investigate the mediating role of organizational maturity on the impact of leadership styles on employees' organizational commitment in Assaluyeh Petrochemical Complex. By studying the related literature in the areas of research, we considered the Avolio and Bass (2004) model (Transformational and Transactional Leadership Styles) in order to determine the leadership style of managers ; and the Allen & Meyer (1990) organizational commitment pattern (emotional commitment, continuous commitment, normative commitment) as a basis for assessing the status of Organizational commitment of staff ; and we applied the human resource capability model (P-CMM) to determine the level of employees' organizational maturity. Data was analyzed using Smart-PLS software. The results showed that there is a significant relationship between leadership style of managers with organizational commitment of employees in Assaluyeh petrochemical complex. Also, organizational maturity has a mediator role in the impact of leadership styles on organizational commitment of employees in Assaluyeh Petrochemical Complex.