Abstract: Hypostichiometric UO_2-x pellets are a novel nuclear fuel used in the special reactors, which is very difficult to be fabricated by the similar processes of the traditional PWR UO_2+x pellets. In this paper, hypostichiometric UO_2-x pellets were fabricated from the blends of UO_2+x and metal uranium. The effects of processes such as uranium particle coating process, uranium content, compacting press and sintering atmosphere on the O/U ratio, density and microstructure of the pellets were studied. The hypostichiometric mechanisms were also investigated. The sintered UO_2-x pellets with a uranium additive of 0, 3% and 6% respectively, have a O/U ratio of 2.010, 1.991 and 1.982, with a average grain size of 10, 15 and 20 μm; 50%U additive into UO_2+x produces O/U ratio of 1.943. Hypostichiometric UO_2-x pellets must be preserved in the sealed chamber full of inert gases.