An enhanced data processing apparatus employing multiple-block based pipeline and an operation method thereof are disclosed. 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 bit stream 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.