A method of fixed bit-rate image compression/decompression based on 2×2 image unit encodes an image that is divided into a plurality of 2×2 coding units. The method determines whether a 2×2 coding unit has one of first to seven man-made patterns. When the 2×2 coding unit has one of the first to seven man-made patterns, a differential error encoding and a quantization and look-up are performed on the 2×2 coding unit for generating a first coded data. Otherwise, the 2×2 coding unit is a natural pattern, and the method sequentially performs a color domain transform, a discrete cosine transform, and a quantization and look-up on the 2×2 coding unit for generating a second coded data.