A method and system for processing billing of including QoS information are provided. In the method, a call is initiated by transmitting/receiving packets directly to/from an opponent terminal after signaling with the opponent terminal using a session initiation protocol through the signaling server. Then, QoS of the call is measured while communicating with the opponent terminal. A billing process is performed of including the QoS measuring result by transmitting the QoS measuring result to the signaling server after the call established to the opponent terminal is terminated.