According to one embodiment, a method for retransmitting a protocol data unit (PDU) in a mobile communication system includes: segmenting a first PDU to form one or more second PDUs if the first PDU does not fit within a PDU size indicated by a lower layer; and transmitting the one or more second PDUs to a receiving side. Segmenting the first PDU includes: mapping only a data field of the first PDU to a data field of the one or more second PDUs; and setting a header of each of the one or more second PDUs. The header of each of the one or more second PDUs includes a field indicating a sequence number of the first PDU.