A multicarrier-based data transmission method and an apparatus for use in a mobile communication system are provided. A Radio Network Temporary Identity (RNTI) reception method of a terminal in a wireless communication system supporting inter-evolved Node B (eNB) carrier aggregation includes receiving cell information on at least one activated cell under control of an eNB, configuring first and second RNTIs allocated by the eNB, monitoring the at least one activated cell for the first RNTI, and monitoring a primary cell among the at least one activated cell for the second RNTI.