An image compression system includes a register, a sensitive detection device, a prediction difference device, a bit-rate control device, a mapping device, and an encoding device, The register temporarily stores plural pixels, which have a current encoding pixel and plural historical decoded pixels. The sensitive detection device analyzes contents of the historical decoded pixels or a previous history data to calculate a prediction value and a current history data. The prediction difference device subtracts the prediction value from the current encoding pixel to generate a prediction difference value. The bit-rate control device adjusts a compression ratio according to a final bits signal so as to output a suggestion bits signal. The mapping device outputs an index based on the prediction difference value, the current history data, and the suggestion bits signal. The encoding device encodes the index to output a bit stream and the final bits signal.