1. A method of decoding a video by using data unit merging, the method comprising: obtaining a merge flag for a prediction unit in the coding unit, from a received bitstream; when the merge flag for the prediction unit indicates a merge mode, obtaining a merge index for the prediction unit from the bitstream, and determining motion information for the prediction unit using the merge index obtained from the bitstream, and when the merge flag for the prediction unit indicates a non-merge mode, obtaining motion information for the prediction unit from the bitstream, wherein the merge index indicates one block among candidate blocks adjacent to borders of the prediction unit, in a merge candidate group and the merge candidate group includes a first candidate block located on a left side of a leftmost block among lower blocks adjacent to a lower border of a current block, and located below a lowermost block among left blocks adjacent to a left border of the current block, and the first candidate block is decoded prior to the prediction unit according to a decoding order, and wherein the merge candidate group includes a predetermined number of available candidate blocks on predetermined locations adjacent to the border of the prediction unit according to an implicitly predetermined order.