A method for performing a random access procedure by a User Equipment (UE) in a mobile communication system includes transmitting a preamble for identifying the UE, through a first message; receiving a second message from an Evolved Node B (ENB) in response to the first message, the second message including uplink (UL) transmission resource information for transmitting at least one of a transparent Radio Resource Control (RRC) message which is an RRC message that a UE having no RRC control connection transmits for the first time, and Buffer Status Report (BSR) information indicating an amount of UL data; setting a format indicator indicating if the transparent RRC message or the BSR information is included in a third message; and generating the third message according to the set format indicator, and transmitting the generated third message according to the UL transmission resource information.