The experimental results regarding to the effects of ultraviolet (UV) light illumination on the characteristics of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFT's) have been presented. The device parameters of a-Si:H TFT, such as threshold voltage, field-effect mobility, and subthreshold slope, have been degraded by electrical stress and visible light illumination, but substantially improved by UV radiation. This may be attributed to an annealing effect on the dangling-bond defects, involving a number of phonons generated by absorption of high energy UV photons in the a-Si:H TFT channel. It has been also observed that the off-current of a-Si:H TFT decreases remarkably while the on-current changes very little. From the experimental results, we report that the improved on/off current ratio of a-Si:H TFT may be achieved by UV radiation.