The present invention relates to a protocol data unit (PDU) reordering method in a wireless communication system. The terminal maintains the number of bytes of PDUs stored in the buffer for reordering PDUs received in a sequence that is changed due to HARQ error correction to thereby prevent overflow of the reordering buffer. When the HARQ function unit transmits a PDU received at the radio access control RAS (S101), the PDU reordering unit of the access terminal sets a current frame number as an arrival frame number of the received PDU (S102). When a current frame number is stored as an arrival frame number of a received frame, the PDU reordering unit compares a sequence number (SN) of a received PDU with a sequence number of a PDU that is about to be reordered (S 103). If the PDU has already been reordered, the received PDU is discarded (S 104). Else, the PDU reordering unit calculates an average size of a buffer that is required for storing the received PDU by using a moving average calculation equation (S 105).