Organizational citizenship behavior (OCB) is one kind of extra-role behavior beyond formal role descriptions which can be conducted to enforce company's competency to copy with external competitive pressures as well as internal flat organizational hierarchies. This research builds a mediating model to examine the mediating effects of organizational justice between discretionary human resource practice (HRP) and organizational citizenship behavior based on the social exchange theory and the organizational justice theory. The research illustrates how the different dimensions of discretionary HRP affects employee's OCB through organizational justice. It statistically tests the data form 227 employees in 10 enterprises with correlation analysis, structural equation modeling and other statistical methods. The results show that all the selective staffing, promotional opportunities, performance management and participate decision making in the discretionary HRP have positive impacts on the OCB. Besides, it indicates that organizational justice mediates the relationship between discretionary HRP and OCB. Thus, organization can promote employee's OCB through enforcing its discretionary HRP as well as enhancing perceived organizational justice. The exposure of the relationships between discretionary HRM practice, organizational justice and OCB is beneficial to build sustainable HRM system and help enterprises to establish sustainable competitive competence.