1. An image decoding method comprising: generating a merge candidate list using available spatial and temporal merge candidates; obtaining motion information using a merge index and the merge candidate list; generating a prediction block using the motion information; inverse-quantizing a quantized block using a quantization parameter to generate a transform block; inverse-transforming the transform block to generate a residual block; and generating a reconstructed block using the prediction block and the residual block, wherein if a current prediction unit is a second prediction unit partitioned by asymmetric partitioning, the spatial merge candidate corresponding to a first prediction unit partitioned by the asymmetric partitioning is set as unavailable, and wherein the quantization parameter is generated using a quantization parameter predictor and a differential quantization parameter, and the quantization parameter predictor is generated by averaging two quantization parameters of a left quantization parameter, an above quantization parameter and a previous quantization parameter if two or more quantization parameters are available.