A writing method of a memory module comprises temporarily storing a piece of 2m-byte data as p characters, wherein each character comprises q bits, and m, p and q are positive integers; rearranging the 2m-byte data to obtain K symbols, wherein each symbol has m bits and K is a positive integer smaller than 2m; encoding the K symbols into a codeword according to a Reed-Solomon (RS)-code algorithm, wherein the codeword comprises N symbols, the codeword has a parity code, the parity code comprises (2T=N−K) symbols, and N and T are positive integers; and writing the p characters and the parity code into a memory-cell array, wherein all the symbols belong to a finite field GF(2m).