Provided are a memory device and a memory module, which perform both an ECC operation and a redundancy repair operation. The memory device repairs a single-bit error due to a ‘fail’ cell by using an error correction code (ECC) operation, and also repairs the ‘fail’ cell by using a redundancy repair operation when the ‘fail’ cell is not repairable by the ECC operation. The redundancy repair operation includes a data line repair and a block repair. The ECC operation may change a codeword corresponding to data per one unit of memory cells including the ‘fail’ cell, and may also change the size of parity bits regarding the changed codeword.