The present invention proposes a method and apparatus for decoding BCH codes and Reed-Solomon codes, in which a modified Berlekamp-Massey algorithm is used to perform the decoding process and the efficiency of the decoder can be improved by re-defining the error locating polynomial as a reverse error locating polynomial, while the operation of the decoding process can be further realized by a common re-configurable module. Furthermore, the architecture of the decoder is consisted of a plurality of sets of re-configurable modules in order to provide parallel operations with different degrees of parallel so that the decoding speed requirement of the decoder in different applications can be satisfied.