Disclosed is a method and an apparatus for reordering received packets in a mobile telecommunication system supporting HARQ so that the problem of order change is solved. The method includes receiving from a receiving-side HARQ entity an input of a packet and processing time spent by the packet going through an HARQ operation; determining if a gap corresponding to at least one missing packet among received packets is detected based on the input packet belonging to the received packets and, when the gap is detected, waiting for the missing packet belonging to the gap to be received for a period of time obtained by subtracting the processing time from a predetermined maximum waiting value; and outputting the received packets to an upper layer in an order when the missing packet belonging to the gap fails to be received before the expiration of the period of time.