A method of processing data in a Medium Access Control (MAC) layer through which at least one first channel is mapped to a second channel in a transmitting end of a wireless communication system is disclosed. More specifically, a MAC layer data block is configured by including at least one higher layer data block received through the at least one first channel and adding a header thereto which includes at least one field which indicates at least two types of information. Furthermore, the MAC layer data block is transferred to a lower layer through the second channel.