The fission-gas released from in-core UO2 pellets was investigated with the helium-swept fuel irradiation facility installed in the Hitachi Training Reactor. It was found that the release rate of fission-gas increased gradually as irradiation continued for 5hr under a constant thermal neutron flux of 1.0×1012 n/cm2·sec and at a fuel temperature of about 150°C. This observation indicates that a recoil-activated release mechanism may be more effective than direct-recoil release in the case of clad fuel in the region of temperature where fission-gas release rate is temperature independent.