Method for decoding chroma image using luma quantization

A method for decoding chroma image, comprising:

• deriving a chroma intra prediction mode of a prediction unit;
• determining a size of a chroma prediction block using luma transform size information;
• generating the chroma prediction block using the chroma intra prediction mode;
• generating a chroma residual 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,
• wherein the chroma quantization parameter is generated using a luma quantization parameter and information indicating the relationship between the luma quantization parameter and the chroma quantization parameter, and
• the luma quantization parameter is generated using a luma quantization parameter predictor and a luma differential quantization parameter, the luma quantization parameter predictor is generated using two quantization parameters which are determined in a predetermined order among a left luma quantization parameter, an above luma quantization parameter and a previous luma quantization parameter when two or three luma quantization parameters are available.