Provided are a method and an apparatus for performing a HARQ on the basis of a polar code. The apparatus generates a response bit to a transport block received by a receiver. The apparatus generates a codeword by encoding predefined information bits with a polar code. The apparatus transmits all or some bits of the codeword to the receiver over a physical channel. The response bit is located at a bit having a higher reliability among the predefined information bits. The size of the codeword is determined by a value which supports the minimum coding rate on the basis of the number of the predefined information bits.