1. A method of decoding a video by using data unit merging, the method comprising: obtaining, from a received bitstream, a skip flag for a coding unit which indicates whether a prediction mode for the coding unit is a skip mode; and when the skip flag indicates the skip mode, obtaining motion information for the coding unit using a merge index obtained from the received bitstream, the merge index indicating one block among final candidates which are searched for in a merge candidate group according to a predetermined order, the merge candidate group comprising a left-lower block located on a left side of a leftmost block among lower blocks adjacent to a lower border of a prediction unit in the coding unit, the left-lower block being located below a lowermost block among left blocks adjacent to a left border of the prediction unit, the left-lower block being decoded prior to the prediction unit according to a decoding order, when the skip flag indicates a non-skip mode, obtaining motion information for the prediction unit, which comprises: obtaining a merge flag for the prediction unit, from the received bitstream; and when the merge flag for the prediction unit indicates a merge mode, obtaining the motion information for the prediction unit using the merge index obtained from the received bitstream, wherein: a maximum number of the final candidates is predetermined, an image is split into a plurality of maximum coding units, according to information about a maximum size of the coding unit, a maximum coding unit, of the plurality of maximum coding units, is hierarchically split into one or more coding units of depths including at least one of a current depth and a lower depth, according to split information, when the split information indicates a split for the current depth, the coding unit of the current depth is split into four rectangular coding units of the lower depth, independently from neighboring coding units, and when the split information indicates a non-split of the current depth, one or more prediction units are obtained from one of the coding units of the current depth.