A user equipment (UE) performing a random access in a mobile communication system includes a transmitter configured to transmit a random access preamble to a network; a receiver receiving a medium access control packet data unit (MAC PDU) including one or more random access responses and a MAC header consisting of one or more sub headers, each of the sub headers respectively corresponding to each of the random access responses; and a processor processing information on a specific random access response among the random access responses based on information on the MAC PDU, wherein all of the sub headers are in a first part of the MAC PDU and all of the random access responses are in a second part of the MAC PDU subsequent to the first part.