A random access (RA) procedure based state transition method in a communication system is disclosed. A terminal in the communication system may comprise transmitting a RA preamble to a base station when an uplink data unit to be transmitted exists in the terminal; receiving a RA response from the base station in response to the RA preamble; and transmitting a message including the uplink data unit to the base station through a resource indicated by the RA response. Therefore, performance of the communication system can be enhanced.