A method for handling segmentation and numbering of protocol data units (PDUs) in a radio link control (RLC) entity of a wireless communications system includes handling a plurality of service data units (SDUs), an SDU of the plurality of SDUs comprising at least a first sequence number (SN) field and a first data field, a PDU of the RLC entity comprising a second data field storing all or segmented data of the first data field, the PDU further comprising a second SN field at least storing a value of the first SN field, and the RLC entity using the second SN field for performing an automatic repeat request, duplicate detection, a sequence number check, in-sequence delivery, etc.