A method for managing HARQ processes in a terminal of a mobile communication system supporting uplink, downlink, and sidelink transmission is provided. The method for managing HARQ processes includes: receiving a data source ID and a HARQ process ID set to at least one of the uplink, downlink, and sidelink transmission through a control channel; searching a new HARQ process ID mapped corresponding to the combination of the data source ID and the HARQ process ID with reference to the HARQ process table; and operating a HARQ process corresponding to the new HARQ process ID.