A method and an apparatus in a mobile communication system are provided. The method by a terminal in the mobile communication system includes receiving, by the terminal, first information for activating a secondary cell (SCell) from a base station, activating, by the terminal, the SCell based on the first information, starting, by the terminal, a timer associated with the SCell, restarting, by the terminal, if second information for the activated SCell is received from the base station, the timer associated with the SCell, and if the timer expires, applying by the terminal, one or more corresponding actions for deactivating the SCell no later than in a predefined subframe. The one or more corresponding actions include at least one of preventing, by the terminal, from monitoring a physical downlink control channel (PDCCH) on the SCell and preventing, by the terminal, from monitoring a PDCCH for the SCell.