The effect of electron beam irradiation on the thermal and mechanical properties of PEEK have been investigated. The films crosslinked with doses more than 20 MGy can be effectively sulfonated in a chlorosulfonic solution, resulting in a crosslinked sulfonated PEEK. The glass transition temperature of irradiated PEEK films increases linearly with absorbed dose. This indicates the formation of crosslinks, these crosslinks prevent crystallization on heating to above Tg. Conversely, the tensile strength and elongation for the irradiated PEEK films decrease with the irradiation dose. It was revealed that a tendency of on embrittlement was affected not only by a crosslinking but also by the orientation of the molecular chains. Furthemore, the decomposition temperatures of the irradiated PEEK films were slightly increased and the residue weight was also increased.