An image decoding method may include: deriving motion information of a first prediction unit determined by a first partitioning mode of a current block and motion information of a second prediction unit determined by a second partitioning mode of the current block; generating a prediction block of the first prediction unit by using the motion information of the first prediction unit and generating a prediction block of the second prediction unit by using the motion information of the second prediction unit; and generating a prediction block of the current block in units of sub-prediction unit determined by combining the first prediction unit and the second prediction unit by using the first prediction block and the second prediction block.