Polyimide films are insulating plastic materials that are useful over a wide temperature range and the most radiation resistant of the polymers. In this paper, mechanical property changes of the polyimide film, KAPTON, caused separately by 14-MeV neutrons and by cobalt-60 gamma rays are examined in detail. Miniature tension specimens of 3-mil-thick film were irradiated at Rotating Target Neutron Source-II and at the Cobalt-60 Pool of Lawrence Livermore National Laboratory at room temperature to a maximum neutron fluence of 1.6 × 10²² n/m² and a maximum gamma dose of 8.6 × 10⁷ Gy, respectively. Neutron fluences greater than 1 × 10²¹ n/m² and gamma dose greater than 2.2 × 10⁷ Gy caused a reduction in total elongation of the film. At our highest tested neutron fluence of 1.6 × 10²² n/m², the elongation dropped to less than one-fifth and the fracture stress dropped to one-third of the unirradiated value. It was found that 14-MeV neutrons were about eight times more effective than cobalt-60 gamma rays in producing mechanical property changes when compared on the basis of absorbed dose. Color changes induced by irradiation were also examined.