A method for receiving a Relay node dedicated Physical Downlink Shared Channel (R-PDSCH) from a base station by a relay node in a radio communication system is disclosed. The method includes receiving, from the base station, a Modulation and Coding Scheme (MCS) index and the number NR-PDSCHPRB1 of first resource blocks to which the R-PDSCH is transmitted, under the assumption that a Relay node dedicated Physical Downlink Control Channel (R-PDCCH) is not transmitted, estimating the number NR-PDSCHPRB2 of second resource blocks to which the R-PDSCH is actually transmitted, using the number NR-PDSCHPRB1 of the first resource blocks and using the number NR-PDCCHPRB of resource blocks to which the R-PDCCH is transmitted, selecting the size of a transport block mapped to the R-PDSCH, using the number NR-PDSCHPRB2 of the second resource blocks and using a transport block index corresponding to the MCS index, and decoding the R-PDSCH, using the size of the transport block and using a modulation order corresponding to the MCS index.