A method of allocating a radio resource in a wireless access system is disclosed. The method includes receiving a control message associated with radio resource allocation for transmitting uplink data to the base station, wherein the control message comprises a first persistent uplink allocation information element comprising first resource allocation information associated with a first resource allocation region for transmitting the uplink data of a mobile station and transmitting an acknowledgment (ACK) message from the mobile station to the base station in response to successfully receiving the first persistent uplink allocation information element.