A handover method of a mobile station in a wireless mobile communication system and a system therefore are provided. The mobile station transmits a handover indication message to a serving base station, receives a ranging response message that comprises authentication information required for the mobile station to authenticate a target base station, receives fast ranging resources and transmits a ranging response acknowledgement (ACK) message, which comprises authentication information required for the target base station to authenticate the mobile station, to the target base station after receiving the fast ranging resources. Accordingly, a handover break time is minimized.