A method of generating a reconstructed block, comprising: deriving an intra prediction mode of a prediction unit using three intra prediction modes which are determined based on left and above intra prediction modes; determining a size of a current block using transform size information; generating a prediction block of the current block according to the intra prediction mode; generating a residual block of the current block according to the intra prediction mode and the size of the current block; and generating a reconstructed block of the current block using the prediction block and the residual block, wherein the size of the current block is equal to or smaller than a size of the prediction unit, and the sizes of the prediction block, the residual block and the reconstructed block are equal to a size of a transform unit, and the transform size information includes one or more split transform unit flags, split_tu_flags, wherein deriving an intra prediction mode of a prediction unit comprises: - constructing a Most Probable Mode, MPM, group which is comprised of three intra prediction modes using the left intra prediction mode and the above intra prediction mode; - determining whether a mode group indicator, which is a flag, indicates the MPM group or not; - setting an intra prediction mode of the MPM group specified by a prediction mode index as the intra prediction mode of the prediction unit if the mode group indicator indicates the MPM group; characterised in that - determining the intra prediction mode of the prediction unit by comparing the prediction mode index with the three intra prediction modes of the MPM group if the mode group indicator does not indicate the MPM group, wherein if one of the left intra prediction mode and the above intra prediction mode is available, the MPM group is comprised of the available intra prediction mode and two additional intra prediction modes, and DC mode and planar mode are set as the two additional intra prediction modes, if the available intra prediction mode is a directional intra prediction mode. deriving an intra prediction mode of a prediction unit using three intra prediction modes which are determined based on left and above intra prediction modes; determining a size of a current block using transform size information; generating a prediction block of the current block according to the intra prediction mode; generating a residual block of the current block according to the intra prediction mode and the size of the current block; and generating a reconstructed block of the current block using the prediction block and the residual block, wherein the size of the current block is equal to or smaller than a size of the prediction unit, and the sizes of the prediction block, the residual block and the reconstructed block are equal to a size of a transform unit, and the transform size information includes one or more split transform unit flags, split_tu_flags, wherein deriving an intra prediction mode of a prediction unit comprises: - constructing a Most Probable Mode, MPM, group which is comprised of three intra prediction modes using the left intra prediction mode and the above intra prediction mode; - determining whether a mode group indicator, which is a flag, indicates the MPM group or not; - setting an intra prediction mode of the MPM group specified by a prediction mode index as the intra prediction mode of the prediction unit if the mode group indicator indicates the MPM group; characterised in that - determining the intra prediction mode of the prediction unit by comparing the prediction mode index with the three intra prediction modes of the MPM group if the mode group indicator does not indicate the MPM group, - constructing a Most Probable Mode, MPM, group which is comprised of three intra prediction modes using the left intra prediction mode and the above intra prediction mode; - determining whether a mode group indicator, which is a flag, indicates the MPM group or not; - setting an intra prediction mode of the MPM group specified by a prediction mode index as the intra prediction mode of the prediction unit if the mode group indicator indicates the MPM group; characterised in that - determining the intra prediction mode of the prediction unit by comparing the prediction mode index with the three intra prediction modes of the MPM group if the mode group indicator does not indicate the MPM group, wherein if one of the left intra prediction mode and the above intra prediction mode is available, the MPM group is comprised of the available intra prediction mode and two additional intra prediction modes, and DC mode and planar mode are set as the two additional intra prediction modes, if the available intra prediction mode is a directional intra prediction mode.