The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting or receiving a MAC PDU in a wireless communication system, the method comprising: setting a LCID field in a MAC subheader to a first value indicating that an eLCID field is included in the MAC subheader when a MAC PDU including the LCID field and the eLCID field is to be generated, setting the eLCID field to a second value identifying a logical channel of a MAC SDU or a type of a MAC CE, and generating and transmitting the MAC PDU including the LCID field and the eLCID field, and the MAC SDU or the MAC CE.