An image decoding method including: obtaining, from a bitstream, information related to a triangle prediction mode for a current block; splitting the current block into two triangular partitions, according to the information related to a triangle prediction mode; generating a merge list for a triangle prediction mode, according to a merge list generation method in a regular merge mode; selecting a motion vector for the two triangular partitions according to information indicating the motion vector from among motion vectors included in the merge list; obtaining, from a reference image, prediction blocks corresponding to the two triangular partitions, based on the motion vector; and reconstructing the current block, based on a final prediction block.