A method for formatting a signal in a mobile communication system by appending a plurality of medium access control headers to a plurality of medium access control-service data units for data transfer between a mobile station and network in the mobile communication system. If the service data units have the same characteristics, a medium access control-protocol data unit is formed by successively coupling the service data units to any one of the medium access control headers. If the service data units have different characteristics, the protocol data unit is formed by sequentially coupling each of the service data units and each of the medium access control headers. The formed protocol data unit is then transformed into a transport block with a predetermined size. A medium access control sublayer formats medium access control-protocol data units according to transport channel characteristics in peer-to-peer communication in such a manner that the protocol data units can have different formats with respect to different transport channels. This makes it possible to provide more efficient functions.