A method of processing control information in a mobile communication system is disclosed, by which an RRC connection setup can be quickly completed and by which control information can be processed without an unnecessary standby of a mobile terminal. The present invention includes the steps of receiving a plurality of protocol data units transmitted plural time from a transmitting side via one common logical channel by an RLC (radio link control) entity operating in a UM (unacknowledged mode) and having a receiving window and a timer, re-ordering a plurality of the received protocol data units using sequence numbers of a plurality of the received protocol data units, the receiving window and the timer, reassembling at least one service data unit by processing a plurality of the re-ordered protocol data units, and delivering the at least one service data unit.