An apparatus and method for avoiding waste of resources by dynamically allocating a length of a Logical Channel IDentification (LCID) of a header included in a Media Access Control (MAC) Protocol Data Unit (PDU) are provided. The apparatus includes a header generator for evaluating a MAC Service Data Unit (SDU) to be multiplexed and for dynamically allocating LCID information according to the evaluated MAC SDU, when radio bearer information is modified. Accordingly, the conventional problem can be solved in which a large number of bits are unnecessarily wasted since the length of the LCID is fixedly allocated or in which logical channels cannot be sufficiently used.