A method of transmitting uplink data by a user equipment (UE) in a wireless communication system includes receiving, by the UE from a eNB, a physical downlink control channel (PDCCH) which is scrambled with a semi-persistent scheduling (SPS)-radio network temporary identifier (RNTI) (SPS-RNTI) and includes a downlink control information (DCI) format 0 for a SPS scheduling assignment; validating, by the UE, the SPS scheduling assignment based on that the SPS-RNTI is not related to the ProSe communication and i) two bits in a first field in the DCI format 0 are set to ‘00’; ii) three bits next to the first field in the DCI format 0 are set to ‘000’; and iii) a most significant bit (MSB) in a second field in the DCI format 0 is set to ‘0’. Further, based on that the SPS-RNTI is related to the ProSe communication, the validating of the SPS scheduling assignment is determined based on that i) the two bits in the first field in the DCI format 0 is set to ‘00’ and ii) the MSB in the second field in the DCI format is set to ‘0’, and the three bits next to the first field are related to a resource information for the ProSe communication.