A method of performing, by a user equipment, UE, an uplink transmission using early data transmission, EDT, in a wireless communication system supporting a narrowband, the method comprising: receiving, from a base station, a control message including first information representing a maximum transport block size, TBS, for the uplink transmission and second information representing whether a selection for a transport block size, TBS, smaller than the maximum TBS is allowed for the uplink transmission; transmitting, to the base station, a message 1 that is a random access preamble for a request of the EDT; receiving, from the base station, a message 2 that is a random access response message including uplink grant, and the uplink grant including resource allocation information for the EDT and a message 3 repetition number; selecting, by the UE, a specific TBS based on the selection for the TBS being allowed; determining, by the UE, a repetition number for a message 3 that is the uplink transmission based on the specific TBS, the maximum TBS included in the control message, and the message 3 repetition number included in the uplink grant; and performing, by the UE, the uplink transmission based on the determined repetition number and the specific TBS, wherein the method further comprises, based on the second information representing that the selection for the TBS smaller than the maximum TBS is allowed for the uplink transmission, receiving, from the base station, third information on a TBS subset that is configured as a part of a TBS set, the TBS set including a plurality of TBSs, and the TBS subset includes at least one TBS smaller than the maximum TBS, and wherein the specific TBS is selected from the TBS subset. receiving, from a base station, a control message including first information representing a maximum transport block size, TBS, for the uplink transmission and second information representing whether a selection for a transport block size, TBS, smaller than the maximum TBS is allowed for the uplink transmission; transmitting, to the base station, a message 1 that is a random access preamble for a request of the EDT; receiving, from the base station, a message 2 that is a random access response message including uplink grant, and the uplink grant including resource allocation information for the EDT and a message 3 repetition number; selecting, by the UE, a specific TBS based on the selection for the TBS being allowed; determining, by the UE, a repetition number for a message 3 that is the uplink transmission based on the specific TBS, the maximum TBS included in the control message, and the message 3 repetition number included in the uplink grant; and performing, by the UE, the uplink transmission based on the determined repetition number and the specific TBS, wherein the method further comprises, based on the second information representing that the selection for the TBS smaller than the maximum TBS is allowed for the uplink transmission, receiving, from the base station, third information on a TBS subset that is configured as a part of a TBS set, the TBS set including a plurality of TBSs, and the TBS subset includes at least one TBS smaller than the maximum TBS, and wherein the specific TBS is selected from the TBS subset.