A method of decoding an image is disclosed. The method comprises deriving an intra prediction mode; generating reference pixels if there exist unavailable reference pixels of a current block; determining whether reference pixels of the current block are filtered or not based on the intra prediction mode and a size of the current block; selecting a filter using the size of the current block and difference between reference pixels and filtering the reference pixels of the current block using the selected filter if it is determined that the reference pixels of the current block are filtered; and generating the prediction block according to the intra prediction mode.