A method for decoding an image according to the present invention comprises the steps of: decoding a residual block by quantizing and inverse transforming an entropy-decoded residual block; generating a prediction block via motion compensation; and decoding an image by adding the decoded residual block to the prediction block, wherein on the basis of the maximum number of motion vector candidates of the motion vector candidate list related to the prediction block, a motion vector candidate list is adjusted by adding a particular motion vector candidate or by discarding a portion from among the motion vector candidates, and in the prediction block generation step, a prediction motion vector of the prediction block is determined on the basis of the adjusted motion vector candidate list. Accordingly, the complexity of arithmetic operations is reduced during encoding/decoding of an image.