A decoding apparatus is used for decoding region of interest (ROI) regions in an image, and includes a storage device, a pre-processing circuit, a decoding circuit, and an information fetching circuit. The pre-processing circuit performs a syntax pre-parsing operation upon a bitstream to obtain necessary information of the ROI regions, and stores the necessary information into the storage device. The decoding circuit performs a decoding operation upon the bitstream to decode the ROI regions, wherein the decoding operation includes syntax parsing of the bitstream. The information fetching circuit reads and analyzes the necessary information, and delivers at least a portion of the necessary information to the decoding circuit. A processing time of obtaining necessary information of one ROI region at the pre-processing circuit overlaps a processing time of decoding another ROI region at the decoding circuit.