A programmable gain MOS amplifier is disclosed. The programmable gain MOS amplifier is capable of increasing its programmable gain linearly in dB unit by increasing its gain level data linearly. The programmable gain MOS amplifier includes a plurality of gain providers for providing predetermined gains respectively, and a plurality of gain tuners. Each of the plurality of the gain tuners is disposed for adjusting the predetermined gain from the corresponding gain provider. Each of the gain tuners includes a gain enabling module and a gain decreasing module. The gain enabling module allows the corresponding predetermined gain to add to the programmable gain of the MOS amplifier. The gain decreasing module declines the corresponding predetermined gain added to the programmable gain of the MOS amplifier.