A system and method for providing Multicast/Broadcast Service (MBS) service to a mobile terminal in an MBS service provisioning system in a Wide Area Network (WAN) network are provided. An authentication server sends a message for requesting resource reservation, in which a Quality-of-Service (QoS) policy is included, when a registration procedure of the mobile terminal is performed. A base station stores the message sent from the authentication server, forwards the message to the mobile terminal, and sets up a channel to the mobile terminal upon receiving from the mobile terminal a request for channel setup based on the QoS policy.