A base station controller (BSC) performs a handoff of a mobile terminal (MT) moving between cells of base transceiver stations (BTSs) in a mobile communication system. If a target BTS is selected as a handoff candidate for the MT as the MT moves from a cell of a source BTS in service toward a cell of the target BTS, the BSC transmits to the target BTS a first message for requesting the target BTS to buffer packets, and transmits packets to be transmitted to the MT to both the source BTS and the target BTS. If the target BTS is determined as a serving BTS for the MT, the BSC transmits to the target BTS a second message including a last sequence number in order to request the target BTS to transmit, to the MT, packets with sequence numbers following the last sequence number of a last packet transmitted from the source BTS to the MT, among the buffered packets.