A method of operating a memory system includes receiving information data corresponding to a second program unit that is a part of a first program unit and a write request for the information data from a host; generating a codeword by performing error correction code (ECC) encoding on the received information data such that a partial parity bit corresponding to the information data among all parity bits of the codeword is updated; and providing a memory device with the generated codeword and a write command regarding the codeword.