Disclosed herein is an image encoding/decoding method. A method of decoding an image includes determining an intra prediction mode of a current block, and generating a prediction block of the current block, by performing prediction based on the intra prediction mode. When the intra prediction mode of the current block is a matrix-based intra prediction (MIP) mode, a length of a reference sample used for intra prediction is derived based on a length of the current block.