A method of allocating a radio resource in a wireless access system includes transmitting, by a base station (BS) to a mobile station (MS), a MAP message comprising a persistent MAP information element, wherein the persistent MAP information element includes resource allocation information for persistently allocating a resource allocation region for communicating data with the MS and acknowledgment (ACK) information indicating an ACK transmission region; waiting for an ACK message from the MS, wherein the ACK message represents a successful reception of the persistent MAP information element; and determining that the persistent MAP information element has not been successfully received by the MS if the ACK message is not transmitted via the ACK transmission region.