Disclosed herein is a method of decoding an image including determining whether a current block is in a bi-directional optical flow (BIO) mode, calculating gradient information of prediction samples of the current block when the current block is in the BIO mode, and generating a prediction block of the current block using the calculated gradient information, wherein the calculating of the gradient information of the prediction samples of the current block includes calculating the gradient information using at least one neighbor sample adjacent to the prediction samples.