The present invention relates to a method in which a receiver receives an MAC (medium access control) PDU (protocol data unit) in a wireless communication system. The method in which the receiver receives an MAC PDU comprises the following steps: receiving an MAC PDU containing a header and a payload from a transmitter, wherein the header includes a first extended header containing a first length field which indicates the length of a service data unit (hereinafter, referred to as “SDU”) or lengths of fragmented SDUs in the MAC PDU; decoding the header; and determining whether the size of the first length field in the extended header is an extended size or a basic size in accordance with the results of decoding.