A method for decoding an image, the method including receiving a bitstream including image information, decoding base block size information on a size of a base block, and a residual quantization parameter of a decoding object unit, from the bitstream, deriving a predicted quantization parameter of the decoding object unit based on the base block size information, deriving a quantization parameter of the decoding object unit based on the predicted quantization parameter and the residual quantization parameter, and decoding the decoding object unit based on the quantization parameter of the decoding object unit.