1. An apparatus of decoding an image, the apparatus comprising: an intra prediction unit for generating a prediction block using an intra prediction mode of a current prediction unit; an inverse scanning unit for inversely scanning significant flags, coefficient signs and coefficient levels according to an inverse scan pattern which is selected based on the intra prediction mode and a size of a transform unit to generate a quantized block; an inverse quantization unit for inversely quantizing the quantized block using a quantization parameter and a quantization matrix to generate a transform block; an inverse transform unit for inversely transform the transform block to generate a residual block; and an adder for adding the prediction block and the residual block to generate a reconstructed block, wherein the quantization parameter is generated using a differential quantization parameter and a quantization parameter predictor, the differential quantization parameter is restored using a bin string indicating an absolute value of the differential quantization parameter and a bin indicating a sign of the differential quantization parameter, if two or more quantization parameters are available among a left quantization parameter, an above quantization parameter and a previous quantization parameter of a current coding unit, the quantization parameter predictor is generated using two available quantization parameters by: if the left quantization parameter and the above quantization parameter are available, an average of the left quantization parameter and the above quantization parameter is set as the quantization parameter predictor, if the left quantization parameter is not available, an average of the above quantization parameter and the previous quantization parameter is set as the quantization parameter predictor, and if the above quantization parameter is not available, an average of the left quantization parameter and the previous quantization parameter is set as the quantization parameter predictor, and the quantization parameter is restored per quantization unit, and a minimum size of the quantization unit is adjusted by a picture parameter set and a coding unit is set as the quantization unit if a size of the coding unit is equal to or larger than the minimum size of the quantization unit.