Disclosed is an upstream access method in an Orthogonal Frequency Division Multiple Access (OFDMA)-based mobile communication system in which a random access method in an OFDMA network includes randomly selecting, by each subscriber device, one of a plurality uplink random access channels, if a random access request to the OFDMA network is transmitted by each subscriber device, accessing through the selected channel if a collision has not occurred in the selected channel, randomly selecting one of the uplink random access channels, if a collision has occurred in the selected channel, and if the number of retries for channel selection is less than a predetermined value, and determining whether a has collision has occurred, accessing through a finally selected channel, if it is determined that a collision has occurred; and randomly selecting one of the uplink random access channels if it is determined that a collision has occurred.