A high-resolution X-ray computed tomography (CT) technique was developed that made it possible to obtain fine X-ray CT images of an irradiated fuel assembly. In addition, the density distributions in an irradiated mixed oxide fuel pellet could be continually measured using the relationship between the densities and the CT values. These results were compared to the results obtained by the metallographic method. It was found that the relative change of radial density distributions in the irradiated fuel pellet can be measured more accurately by the X-ray CT technique than by metallographic examination.