A method of transport block retransmission in a wireless communication system employing carrier aggregation is disclosed. A method of retransmitting transport blocks in a base station of a wireless communication system employing carrier aggregation comprises mapping each of a plurality of transport blocks to each of a plurality of component carriers using a first mapping pattern and transmitting the plurality of transport blocks to a mobile station; recognizing that the plurality of transport blocks are not transmitted successfully; and mapping each of the plurality of transport blocks to each of the plurality of component carriers using a second mapping pattern and retransmitting the plurality of transport blocks to the mobile station.