Determine the right price for one application usage is a core function of Rating Module in real time charging. Telecom operators base their tariff plans and promotions for services on this function to implement. Hence, rating plays a very important role to implement flexibility in operators billing and charging platform and to maintain their investments without penalizing customer according to service delivery. In the context of all-IP paradigm with the IP Multimedia System introduction, and the consequent revolution in terms of applications more perceived as multimedia sessions evolving many types of media and many stakeholders at the same time, the concept of rating evolved as well to tackle the challenge of multimedia complex applications and quality of service in packet switched networks. Consequently, the function of rating these applications depends on many service, network and commercial parameters which make an accurate real time charging a complex task. In order to handle this complexity, the present contribution suggests an enhancement of rating parameters in the online charging system of IMS. Also, it defines a modeling of rating function of the online charging system. This modeling aims to define the main blocks of this function and the interaction with network, business information and user information