A method for processing authentication of a mobile station in a mobile communication system comprises determining whether an authentication state is inactivated, and requesting registration. The method also comprises receiving a result message from a network, and activating the authentication state according to the result message. The authentication state may be checked using a success flag or a fail flag. The success flag may be activated when the result message is a success message. The fail flag may be activated when the result message is a fail message. The activating the authentication state may comprise activating an authentication success state in response to activation of the success flag, and activating an authentication fail state in response to activation of the fail flag. The method may further comprise entering a lock state when the authentication state is activated as an authentication fail state.