The parallel decoding method of a parallel concatenation of multiple codes is well known. In this paper, we present a new serial decoding method. The iterative gain in this method is always one. Therefore, this method does not need optimization of the iterative gain by using simulated annealing like the parallel decoding method. Though it is simpler than the parallel decoding method in calculation, it gives the same performance. We also use Pearls propagation algorithm to show the appropriateness of the serial decoding method.