1. A method of decoding a video signal under a merge mode in a decoding apparatus, the method comprising: obtaining a spatial merge candidate of a current block from a neighboring block adjacent to the current block, motion prediction related information of the spatial merge candidate being determined based on the neighboring block obtaining a collocated reference index for identifying a collocated picture among a plurality of previously decoded pictures; determining the collocated picture having a different temporal order from a current picture including the current block based on the collocated reference index, wherein one of a plurality of decoded pictures specified by the collocated reference index is determined to be the collocated picture; determining a collocated block included in the collocated picture based on whether or not a boundary of the current block adjoins a boundary of a largest coding unit; obtaining a temporal merge candidate of the current block from the collocated block in the collocated picture, motion prediction related information of the temporal merge candidate being determined based on the collocated block; generating a merge candidate list including the spatial merge candidate and the temporal merge candidate; obtaining motion information of the current block based on the merge candidate list; obtaining prediction samples of the current block based on the motion information of the current block; and obtaining reconstruction samples of the current block by using the prediction samples and residual samples of the current block.