disclosed are an intra prediction method of a chrominance block using a luminance sample and an apparatus using the same . an image decoding method comprises the steps of : calculating an intra prediction mode of a chrominance block on the basis of an lm mapping table when the chrominance block uses an lm ; and generating a prediction block for the chrominance block on the basis of the calculated intra prediction mode of the chrominance block . when intra prediction mode information of chrominance blocks are decoded , mutually different tables are used depending on whether or not an lm is used , so that encoding and decoding can be performed without an unnecessary waste of bits .