1. An image decoding method using a merge mode, the method comprising: constructing a merge candidate list using available spatial and temporal merge candidates; obtaining motion information using a merge index and the merge candidate list; deriving a prediction block using the motion information; inverse-quantizing a quantized block using a quantization parameter and a quantization matrix to generate a transform block, and inverse-transforming the transform block to generate a residual block; and generating a reconstructed block using the prediction block and the residual block, wherein the quantization parameter is generated per a quantization unit, and a size of the quantization unit is adjusted per a picture, wherein the quantization parameter is generated using a differential quantization parameter and a quantization parameter predictor, and the quantization parameter predictor is derived using two available quantization parameters of a left quantization parameter, an above quantization parameter and a previous quantization parameter, wherein the quantization parameter predictor is derived using the left quantization parameter and the above quantization parameter when all of the left quantization parameter, the above quantization parameter and the previous quantization parameter are available, and wherein the differential quantization parameter is obtained from a received bitstream.