Provided are a method and an apparatus for decoding an image by accessing a memory by a block group. The method comprises checking information regarding a size of one or more blocks included in a bitstream of an encoded image, determining whether to group one or more blocks for performing decoding, based on the information regarding the size of the one or more blocks, setting a block group including one or more blocks based on the information regarding the size of the one or more blocks, and accessing a memory by the block group to perform a parallel-pipeline decoding process by the block group.