There are provided an apparatus and method for retransmitting data according to a radio link protocol in a mobile communication system. In the method of requesting frame retransmission, a receiver stores the sequence numbers of a plurality of RLP (Radio Link Protocol) frames that are not received from a transmitter and transmits a retransmission request frame including fields that indicate the sequence numbers. Then, the transmitter sequentially transmits the requested RLP frames in the order of the sequence numbers to the receiver. The receiver compares the stored sequence numbers with the sequence numbers of received RLP frames and if it receiver fails to receive one of the requested RLP frames, the receiver requests retransmission of the lost RLP frame upon receipt of an RLP frame of a higher sequence number than the sequence number of the lost RLP frame.