Abstract: Based upon JPEG still image compression standard, an expanded adaptive quantizer is presented in this paper. According to the visual perception, visual activities are analyzed to exploit local perceptual characteristic in the MCU. The adaptive quantization factor is decided based on the MCU activity function, and a brightness masking operator is introduced to modify the quantization scales. Experiment result shows that the coding distortion is uniformly decreased with the proposed quantizer, picture quality is improved, and a better coding effect is acquired.