A method of data retransmission in a multi carrier system, the method comprising: receiving a first grant message including first resource allocation information for an uplink data transmission and a first new data indicator, NDI, indicating that the uplink data is new data; transmitting the uplink data using the first resource allocation information; receiving an acknowledgement message representing whether the uplink data is successfully received; maintaining the uplink data in a buffer until a predetermined condition is satisfied even though the acknowledgment message is a positive acknowledgment, ACK, signal, in order to retransmit the uplink data if a NACK-to-ACK error is detected; the method being characterized in that it further comprises: adaptively retransmitting the uplink data by using second resource allocation information, if a second grant message is received after the acknowledgement message has been received, the second grant message including the second resource allocation information for the uplink data retransmission and a second NDI indicating a retransmission; or non-adaptively retransmitting the uplink data by using the first resource allocation information, if the second grant message is not received after the acknowledgement message has been received and the acknowledgment message is a negative acknowledgment, NACK, signal. receiving a first grant message including first resource allocation information for an uplink data transmission and a first new data indicator, NDI, indicating that the uplink data is new data; transmitting the uplink data using the first resource allocation information; receiving an acknowledgement message representing whether the uplink data is successfully received; maintaining the uplink data in a buffer until a predetermined condition is satisfied even though the acknowledgment message is a positive acknowledgment, ACK, signal, in order to retransmit the uplink data if a NACK-to-ACK error is detected; the method being characterized in that it further comprises: adaptively retransmitting the uplink data by using second resource allocation information, if a second grant message is received after the acknowledgement message has been received, the second grant message including the second resource allocation information for the uplink data retransmission and a second NDI indicating a retransmission; or non-adaptively retransmitting the uplink data by using the first resource allocation information, if the second grant message is not received after the acknowledgement message has been received and the acknowledgment message is a negative acknowledgment, NACK, signal. adaptively retransmitting the uplink data by using second resource allocation information, if a second grant message is received after the acknowledgement message has been received, the second grant message including the second resource allocation information for the uplink data retransmission and a second NDI indicating a retransmission; or non-adaptively retransmitting the uplink data by using the first resource allocation information, if the second grant message is not received after the acknowledgement message has been received and the acknowledgment message is a negative acknowledgment, NACK, signal.