The present invention relates to a mobile communication system, and in particular, to a method and apparatus for transmitting and receiving a PDU including an Extension Bit field and a Length Indicator field in a UE or a Node B. A method for transmitting a PDU generated with more than two SDUs and a header having information on each SDU in a transmitter of a mobile communication system is provided. The method includes generating, in the header, a Length Indicator (LI) indicating an end point of each of remaining SDUs except for the last SDU and an Extension Bit indicating the presence/absence of another LI after each LI, and generating the header so that the LI succeeds the Extension Bit associated therewith; generating the PDU with the generated header and the SDUs; and transmitting the PDU.