The main function of a rating function in an online charging system is to perform real-time calculations of the tariff. In the context of the IP multimedia subsystem, and the consequent revolution in terms of services and applications, quality of service and network performance play an important role in application delivery and user experience. Consequently, a rating function that integrates these new parameters is important to address the challenge of an accurate real-time charging of applications in IP multimedia subsystem environments. The present contribution suggests an approach for modeling rating functions in the online charging system of IMS. Furthermore it defines the interactions between the rating function and service elements and network elements and business actors