A method for changing a subscription status of a service in a mobile communication system, and a system thereof. A method is provided for changing a subscription status of a service by a terminal in a mobile communication system. The method includes transmitting a pause request message, in which a pause period of the subscription status is included, to a network entity; receiving, from the network entity, a pause response message including therein a possible pause period for the pause period included in the pause request message, and a trigger for changing validity for a Long-term Key stored in the terminal; and changing validity for the Long-term Key using the trigger for the possible pause period included in the pause response message.