The radiation effect on then‐channel polycrystalline silicon (polysilicon) thin‐film transistors has been investigated. It is found that for an unhydrogenated device, the irradiation of Co60with a total dose of 1 Mrads caused a negative threshold‐voltage (Vth) shift and a slight subthreshold‐swing (S) degradation, while for a hydrogenatedn‐channel device, the same irradiation results in a positiveVthshift and a seriousSdegradation. It is also found that the radiation hardness of the hydrogenated devices can be improved somewhat by a simple irradiation‐then‐hydrogenation treatment. ©1995 American Institute of Physics.