Disclosed is a method and an apparatus for transmitting and receiving data via a MAC protocol in a mobile communication system. The method includes inputting at least one Service Data Unit (SDU) containing transmission data through a corresponding logical channel and generating at least one first Protocol Data Unit (PDU) that includes said at least one SDU without including multiplexing information for identification of the logical channel, by a first transmission entity; acquiring the first PDU and generating a second PDU including the first PDU in a payload of the second PDU, by a second transmission entity that operates between the first transmission entity and a physical layer; inserting the multiplexing information for identification of the logical channel corresponding to said at least one first PDU into header information of the second PDU; and transmitting the second PDU through the physical layer. The method can reduce load due to additional processing, such as a bit operation or memory copying, in a receiver requiring high speed data transmission.