An apparatus of decoding a video in the unit of blocks, includes: a size setting unit to decode information on a picture size of the video and information on available block sizes from a bitstream, set an initial size of a current block to be decoded to a largest block size determined by the information on the available block sizes, determine a location of the current block within a picture, identify whether the current block is fully located inside the picture, based on the picture size, the size of the current block and the location of the current block, and reset the size of the current block to a smaller size among the available block sizes, when the current block is not fully located inside the picture; and a decoding unit to decode the current block in the reset size.