A method for decoding an image is discussed. The method can include generating a residual block; reconstructing an intra prediction mode group indicator and a prediction mode index of a prediction unit; constructing a first group including three intra prediction modes using available intra prediction modes of left and top blocks of the prediction unit; determining the intra prediction mode corresponding to the prediction mode index in the first group as the intra prediction mode of the prediction unit when the intra prediction mode group indicator indicates the first group; generating a prediction block on the basis of the determined intra prediction mode of the prediction unit; generating a reconstructed block using the residual block and the prediction block; and performing deblocking filtering on a reconstructed picture.