Disclosed is an apparatus and method for an MBS service in a BWA system. An apparatus of a base station includes a time controller, a CAC processor, and a buffer controller. The time controller manages a broadcast start time and a broadcast end time and generates a broadcast start message at a predetermined time before the actual broadcast start time. Upon receipt of the generated broadcast start message from the time controller, the CAC processor calculates a capacity decrease due to broadcast service and subtracts the capacity decrease from the current available capacity to update the available capacity. Upon receipt of the generated broadcast start message from the time controller, the buffer controller controls the buffer occupation of unicast traffic in order to provide the buffer space required for a broadcast service.