A method for requesting retransmission of high-speed packet data in a receiving Automatic Retransmission reQuest (ARQ) entity of a mobile communication system that simultaneously performs Hybrid Automatic Retransmission reQuest (HARQ) and ARQ. The method includes, upon receipt of a packet, determining a sequence number of the received packet, and determining if there is at least one missing packet preceding the received packet; if there is at least one missing packet, driving a first timer which is set to a time required when a number of retransmission attempts reaches a predetermined maximum number of HARQ retransmissions, and monitoring receipt of the missing packet; and upon expiration of the timer, sending to a transmitting ARQ entity an Acknowledgement (ACK) including a last sequence number among sequence numbers of normally received consecutive ARQ packets.