The physical properties of γ-ray-irradiated and thermally aged Poly(ether ether ketone) (PEEK) have been investigated. To evaluate the degradation level of PEEK, analyses of the mechanical, electrical, and dielectric relaxation properties were carried out for accelerated-aged PEEK. Studies of the temperature dependency of the dielectric properties indicated that the glass transition temperature of the aged PEEK increased after the radiation and thermal aging. The frequency dependency of the dielectric properties implied that the magnitude of the induced dipoles and ions increased upon increasing the radiation doses and thermal aging. The values of the relaxation intensity calculated using Cole-Cole’s circular arc could be useful for evaluation of the degradation level of PEEK.