A method of performing random access in a broadband wireless access system is disclosed. The method includes performing first random access through at least one of a plurality of random access channels, receiving information about the number of access-failed channels among the plurality of random access channels from a base station, when the first random access fails, and performing second random access according to a probability value calculated based on the received information. The present invention increases utilization of random access channels and can effectively schedule an uplink bandwidth, thereby providing an efficient ranging method.