A method and apparatus are provided for supporting a large Service Data Unit (SDU) in a receiver of a mobile communication system. The method includes receiving a Radio Link Control (RLC) Protocol Data Unit (PDU) from an upper layer; checking a Length Indicator (LI) included in an RLC header of the RLC PDU; if the LI is configured as “0”, determining that the received RLC PDU includes an RLC SDU of 2047 bytes, and if the LI is configured as a value other than “0”, determining that the received RLC PDU includes an RLC SDU of a size corresponding to the value of the LI; and reforming the RLC SDU, based on the LI.