A method for decoding chroma image is discussed. The method includes deriving a chroma intra prediction mode of a prediction unit; determining a size of a current chroma block using luma transform size information; generating a chroma prediction block of the current chroma block using the chroma intra prediction mode; generating a chroma residual block of the current chroma block using the chroma intra prediction mode and a chroma quantization parameter; and generating a chroma reconstructed block adding the chroma prediction block and the chroma residual block.