Provided is an apparatus and method for updating an Authorization Key (AK) of a mobile station in an idle mode in a wireless communication system. The method includes transmitting a first location update request to a base station, receiving a response to the request from the base station, the response to the first location update request including a NONCE_BS, wherein the NONCE_BS is a random number associated with the base station and used for updating the AK, generating a NONCE_MS, wherein the NONCE_MS is a random number used for updating the AK, generating the AK using the NONCE_BS and the NONCE_MS, transmitting a second location update request to the base station, receiving a response to the second location update request from the base station, the response to the second location update request including the NONCE_BS and the NONCE_MS, and confirming the AK based on the NONCE_BS and the NONCE_MS.