It is found that medium energy X-ray irradiation causes greater, more than one order of magnitude, damages than high energy X-ray irradiation in Si MOSFETs. This phenomenon could be attributed to larger X-ray absorption by photoelectric effect in medium energy region, and smaller X-ray absorption caused by the Compton effect in high energy region. X-ray energy dependent radiation damages, therefore, should be seriously considered to evaluate exact radiation hardness of the Si MOS devices.