A method of transmitting data in a wireless access system and a packet retransmitting method therein are disclosed. The present invention includes receiving an initial control message including information required for a packet retransmission from a base station, transmitting the packet to be provided with a service from the base station, and if a NACK (non-acknowledgement) signal is received from the base station, retransmitting the packet corresponding to the NACK signal to the base station using the information required for the packet retransmission and included in the initial control message. Therefore, since a separate control message needs not to be transmitted in case of packet retransmission, it is able to save radio resources and reduce an overhead by retransmitting a packet via a previously allocated retransmission region without the separate control message.