The present disclosure relates to communication methods and systems for converging a 5th-Generation (5G) communication system for supporting higher data rates beyond a 4th-Generation (4G) system utilizing technology for Internet of Things (IoT). The present disclosure is applicable to intelligent services utilizing 5G communication technology and IoT-related technology, such as smart home, smart building, smart city, smart car, connected car, health care, digital education, smart retail, security and safety services. A Secondary Cell (SCell) method and apparatus for activating an SCell are provided for use in a mobile communication system supporting dual connectivity. The method includes receiving a control message instructing activation of at least one SCell, determining whether the SCell is a primary SCell (pSCell) based on the control message, monitoring, when the SCell is the pSCell, a Physical Downlink Control Channel (PDCCH) of the pSCell, and reporting, after starting PDCCH monitoring, Channel Status Information (CSI) for the SCell.