A method for decoding an image, the method comprising: deriving initial motion vectors from a merge candidate list of a current block; deriving refined motion vectors using the initial motion vectors; and generating a prediction block of the current block using the refined motion vectors, wherein the step of deriving the initial motion vectors includes: deriving the initial motion vectors based on initial motion vector index information when the initial motion vector index information is entropy decoded in a bitstream, wherein the merge candidate list includes a L0 list and a L1 list, and wherein each of the L0 list and L1 list includes motion vectors and reference picture indexes, characterized in that the step of deriving the initial motion vectors further includes: deriving the initial motion vectors by assigning the first merge candidate of the merge candidate list when the initial motion vector index information is not entropy decoded, wherein if a motion vector and reference picture index for the L0 list exist while a motion vector and reference picture index for the L1 list do not exist in the merge candidate list for a specific value of the initial motion vector index information, the first available motion vector and reference picture index in the L1 list are used as the initial motion vector and reference picture index for the L1 list for the specific value of the initial motion vector index information. deriving initial motion vectors from a merge candidate list of a current block; deriving refined motion vectors using the initial motion vectors; and generating a prediction block of the current block using the refined motion vectors, wherein the step of deriving the initial motion vectors includes: deriving the initial motion vectors based on initial motion vector index information when the initial motion vector index information is entropy decoded in a bitstream, wherein the merge candidate list includes a L0 list and a L1 list, and wherein each of the L0 list and L1 list includes motion vectors and reference picture indexes, characterized in that the step of deriving the initial motion vectors further includes: deriving the initial motion vectors by assigning the first merge candidate of the merge candidate list when the initial motion vector index information is not entropy decoded, wherein if a motion vector and reference picture index for the L0 list exist while a motion vector and reference picture index for the L1 list do not exist in the merge candidate list for a specific value of the initial motion vector index information, the first available motion vector and reference picture index in the L1 list are used as the initial motion vector and reference picture index for the L1 list for the specific value of the initial motion vector index information. deriving the initial motion vectors based on initial motion vector index information when the initial motion vector index information is entropy decoded in a bitstream, wherein the merge candidate list includes a L0 list and a L1 list, and wherein each of the L0 list and L1 list includes motion vectors and reference picture indexes, characterized in that the step of deriving the initial motion vectors further includes: deriving the initial motion vectors by assigning the first merge candidate of the merge candidate list when the initial motion vector index information is not entropy decoded, wherein if a motion vector and reference picture index for the L0 list exist while a motion vector and reference picture index for the L1 list do not exist in the merge candidate list for a specific value of the initial motion vector index information, the first available motion vector and reference picture index in the L1 list are used as the initial motion vector and reference picture index for the L1 list for the specific value of the initial motion vector index information. deriving the initial motion vectors by assigning the first merge candidate of the merge candidate list when the initial motion vector index information is not entropy decoded, wherein if a motion vector and reference picture index for the L0 list exist while a motion vector and reference picture index for the L1 list do not exist in the merge candidate list for a specific value of the initial motion vector index information, the first available motion vector and reference picture index in the L1 list are used as the initial motion vector and reference picture index for the L1 list for the specific value of the initial motion vector index information.