The change in thermal properties induced by irradiation of 10 MeV H+, 20 MeV He2+ and 2 MeV electron were studied for noncrystalline and crystalline poly(ether-ether-ketone) (PEEK), PEEK-a and PEEK-c, by differential scanning calorimetry (DSC). It was revealed from the analysis of thermal parameter changes that crosslinking proceeds by irradiation in all cases studied. The comparison of effects between ion and electron irradiations led to a conclusion that probability of crosslinking in the ion irradiation is extremely large compared with that in the electron irradiation. It was clarified from isothermal crystallization that ion gives more severe damage to crystallite than electron. The tensile properties for PEEK-a were varied in accord with difference in thermal properties between ion and electron irradiations, whereas for PEEK-e the difference in those between ion and electron irradiations was scarcely observed.