A method for encoding an image includes determining an intra prediction mode of a current block; generating a prediction block of the current block according to the intra prediction mode; generating a residual block by calculating a difference between the current block and the prediction block; transforming the residual block to generate a transform block; quantizing the transform block to generate a quantized block; and entropy-coding coefficients of the quantized block, wherein the generating the prediction block of the current block according to the intra prediction mode includes: generating reference pixels on an unavailable position if there exist unavailable reference pixels of the current block; determining whether reference pixels of the current block are filtered or not based on the intra prediction mode and a block size of the current block; filtering the reference pixels of the current block using a filter if it is determined that the reference pixels of the current block are filtered, wherein the reference pixels are adaptively filtered based on the block size of the current block, wherein the filter is not applied if it is determined that the size of the current block is smaller than a first size, wherein if a number of intra prediction modes to which the filter is applied is increased, it is determined that the size of the current block is larger than the first size and smaller than a second size, and wherein the reference pixels of the current block are not filtered in a horizontal mode because the image is correlated horizontally.