A Maximum A Posteriori (MAP) decoder and a MAP decoding method are provided. The MAP decoder includes a first metric operation unit, a first bit-width control unit, a second metric operation unit, a Log Likelihood Ratio (LLR) operation unit, and a second bit-width control unit. The first metric operation unit outputs a first metric data using an input data. The first bit-width control unit controls a bit-width of the first metric data according to a modulation scheme of the input data. The second metric operation unit outputs a second metric data using the first metric data having the controlled bit-width. The LLR operation unit outputs LLR data using the second metric data. The second bit-width control unit outputs decoding data by re-controlling the bit-width of the LLR data.