Abstract: U-Mo alloy is one of candidates of metallic fuel for advanced nuclear reactor due to its good irradiation behavior. Reasonably analysis suggests that the irradiation swell of U-Mo alloy can be decreased by introducing homogenously distributed voids, because they can accommodate gaseous fission products. The process of preparing low density U-Mo alloy by powder metallurgy was described, including preparing low density bulk materials by pressing and vacuum sintering. A serial of U-10%Mo alloys with different densities were obtained and the microstructure was analyzed by optical micro-scopy (OM) and scanning electron microscopy (SEM). It is proved that the density of sample increases with sinister time under 1 100 ℃. The void ratio can be controlled by adjusting sinister process conveniently.