A method for decoding an image, according to the present invention, comprises the steps of: determining whether a coding block is divided into a first prediction unit and a second prediction unit; determining a partition type of the coding block when determining that the coding block is divided; deriving first motion information about a first prediction unit and second motion information about a second prediction unit within the coding block; and acquiring a prediction sample within the coding block on the basis of the first motion information and the second motion information.