Abstract: The degradation of Zn O caused by Ultraviolet irradiation in vacuum is studied in this pa per. The objective ofthis study to understand the degradation process in order to produce coatings that arem ore resistant to radiation dam age. The absorption of free carriers produced due to the change of photocon ductivity under the vacuum ultraviolet irradiation is discussed and analyzed. A degradation m echanism in Zn O is proposed in this paper. It can be related to the desorption ofchem isorbed oxyen. The desorption ofchem isorbed oxygen leads to the production of oxygen vacancies in Zn O w hich produce free electrons inconduction band. The degradation of optical properties of Zn O can be attribu ted to the absorption offreeelectrons in near infrared region.