A method to transmit data by a user equipment through an uplink channel in a mobile communication system, the method comprising: receiving an uplink grant, UL Grant, at the user equipment from a base station on a specific message; the method being characterized in that it includes the steps of: - transmitting, by the user equipment to the base station in accordance with the UL Grant received on the specific message, data of a message 3, Msg3, buffer, when the UL Grant is received on the specific message, and the specific message is a random access response message and there is data stored in the Msg3 buffer of the user equipment, - transmitting, by the user equipment to the base station, new data, when the UL Grant is received on the specific message, and the specific message is other than the random access response message and there is data stored in the Msg3 buffer of the user equipment, - transmitting, by the user equipment to the base station, new data, when the UL Grant is received on the specific message, and the specific message is other than the random access response message and there is no data stored in the Msg3 buffer of the user equipment, and - transmitting, by the user equipment to the base station, new data, when the UL Grant is received on the random access response message, and the specific message is the random access response message and there is no data stored in the Msg3 buffer of the user equipment. receiving an uplink grant, UL Grant, at the user equipment from a base station on a specific message; the method being characterized in that it includes the steps of: - transmitting, by the user equipment to the base station in accordance with the UL Grant received on the specific message, data of a message 3, Msg3, buffer, when the UL Grant is received on the specific message, and the specific message is a random access response message and there is data stored in the Msg3 buffer of the user equipment, - transmitting, by the user equipment to the base station, new data, when the UL Grant is received on the specific message, and the specific message is other than the random access response message and there is data stored in the Msg3 buffer of the user equipment, - transmitting, by the user equipment to the base station, new data, when the UL Grant is received on the specific message, and the specific message is other than the random access response message and there is no data stored in the Msg3 buffer of the user equipment, and - transmitting, by the user equipment to the base station, new data, when the UL Grant is received on the random access response message, and the specific message is the random access response message and there is no data stored in the Msg3 buffer of the user equipment. - transmitting, by the user equipment to the base station in accordance with the UL Grant received on the specific message, data of a message 3, Msg3, buffer, when the UL Grant is received on the specific message, and the specific message is a random access response message and there is data stored in the Msg3 buffer of the user equipment, - transmitting, by the user equipment to the base station, new data, when the UL Grant is received on the specific message, and the specific message is other than the random access response message and there is data stored in the Msg3 buffer of the user equipment, - transmitting, by the user equipment to the base station, new data, when the UL Grant is received on the specific message, and the specific message is other than the random access response message and there is no data stored in the Msg3 buffer of the user equipment, and - transmitting, by the user equipment to the base station, new data, when the UL Grant is received on the random access response message, and the specific message is the random access response message and there is no data stored in the Msg3 buffer of the user equipment.