A block decoding method and system capable of decoding and outputting data in a longitudinal direction, which obtains a start address in an input compressed datastream for each block of a decompressed image by a pre-scanned compressed datastream such that the system can obtain 90.degree.-rotated blocks by performing a Huffman decoding in a longitudinal direction, a 90.degree.-rotated zigzag arrangement, an inverse quantization and an IDCT. Since the decoding direction is identical to the output direction of a printer, only a desired column of blocks of the decompressed image is temporarily stored for output and accordingly required memory size is relatively reduce.