An image decoding method in a merge mode, can include constructing a merge candidate list using available spatial and temporal merge candidates, and deriving motion information using a merge index and the merge candidate list; generating a prediction block using the motion information; generating a quantization block by inverse-scanning quantized coefficient components according to a diagonal scan, inverse-quantizing the quantization block using a quantization parameter and a quantization matrix, and inverse transforming the inversely-quantized quantization block to generate a residual block; and deriving a reconstructed block using the prediction block and the residual block.