A secondary carrier activation/deactivation method for activating/deactivating secondary carriers in addition to a primary carrier is provided for a mobile communication system supporting carrier aggregation. The method includes identifying, when a secondary carrier control message is received from a base station, carriers to be activated are deactivated, determining, when the message includes a secondary carrier activation command, whether a first time point has arrived, and executing, when the first time point arrives, predetermined first operations. The present invention is advantageous to avoid malfunctions in communication system supporting carrier aggregation by defining UE operations to be taken in SCell activation and deactivation.