Abstract: As one of the high density of uranium fuel, the uranium nitride has become a pile of future space power, space nuclear power and rocket’s fuel. In this paper, uranium nitride powder and pellet with high density were prepared by hydrogenation-dehydrogenation, nitriding, denitriding and hot pressing sintering process, the effect of powder size on pellet property was investigated. The results show that phase of powder is uranium nitride, and morphology is irregular and non-spherical. It is beneficial to the interfiling and mechanical meshing of powder during pellet preparation, and phases of all the pellets which prepared by powder of different sizes are uranium nitride. The density, uranium density and crushing strength of pellets prepared by powder with mesh count -100-+200 are highest, 13.78 g/cm³ (96.23%T.D.), 12.99 g/cm³ and 175.4 MPa respectively. Mesh count -50-+100 takes second place and mesh count -200 is the worst. And for thermal diffusion coefficients of uranium nitride pellets prepared by powder of mesh count -100-+200 and -50-+100, pellets prepared by powder with mesh count -100-+200 is higher than mesh count -50-+100, and thermal expansion coefficient is close.