The effect of UV irradiation on ZnO thin film based gas sensor was investigated. Zinc oxide thin films were deposited on an alkali free glass substrate by magnetron sputtering system using zinc target. The UV irradiation of the ZnO thin films was measured to understand the change of microstructure, electrical properties, optical properties and gas sensing characteristics. The X-ray diffraction patterns and SEM images revealed that the films have a nanocrystalline structure. The optical properties of ZnO films were not affected by the UV irradiation significantly. The gas sensing behavior of zinc oxide thin films were enhanced by UV irradiation for a shorter period whereas sensing characteristics were degraded for a longer irradiation period. It was also observed that the dependence of gas sensing characteristics was correlated with the change of electrical properties and crystallinity of films.