A method of compressing and decompressing images is disclosed for applications in compression and decompression chips with the JBIG standard. The pipeline of computing a pixel is divided into three parts: memory access, numerical operations, renormalization and byteout/bytein. Each steps takes a work cycle; therefore, three pixels are processed in parallel at the same time. In comparison, the work cycle of the prior art without pipeline improvement is longer. This method can effectively shorten the work cycle of each image data process, increasing the speed of compressing and decompressing image data.