The release rate of fission-gas from U02 was continuously measured during irradiation in the Hitachi Training Reactor. The U02 specimen was heated electrically in in-core assemblies by tungsten heaters, either arranged axially transversing the specimen (producing radial temperature gradient) or cylindrically outside the specimen (uniform heating). In a case of the axially heated annular U02 pellets with radial temperature gradient, the rate of fission-gas release increased with time under constant temperature and flux. This experimental result was explained by considering as fission gas release mechanism a combination of diffusion and pore migration processes. On the other hand, when U02 pellets were heated isothermally up to 2,000°C, the fission-gas release rate could be explained in terms of diffusion process alone. No effect of equiaxial grain growth was observed on the rate of fission-gas release.