A method of generating protocol data units in a radio link control layer, which is set to an unacknowledged mode or an acknowledged mode, is disclosed. After producing a payload unit by segmenting and/or concatenating one or more service data units received from a higher layer, a protocol data unit including a sequence number corresponding to the payload unit and a protocol data unit including the payload unit itself are generated and transmitted separately to a media access control layer through a pair of different channels. Therefore, the PDUs having the sequence number can be transmitted in a low rate in order to reduce the its error rate, and the other PDUs having data can be transmitted in a higher rate to increase the data processing rate.