Abstract: The stability of γ phase of U-Mo and U-Mo-X (X＝Ti, V, Si) alloys, diffusion couples of U-Mo/Al and U-Mo-X/Al was studied, and effects of alloying elements and annealing process on γ phase stability were investigated. The higher Mo content is, the more stable γ phase is. The γ phase of U-6.5Mo-0.5Si alloy is stable because the mixture enthalpy of U-Si is low, but Si addition into U-Mo alloy leads to formation of brittle phase USi_x. The γ phase of U-6.5Mo-0.5Ti or U-6.5Mo-0.5V alloys is unstable because the mixture enthalpy of Mo-Ti or Mo-V is low, and this leads to formation of solid -solution or compounds and Mo dilution in γ phase. As annealing temperature increasing from 500 ℃ to 600 ℃, γ phase decreases by eutectoid decomposition, α phase increases, and some new Al-rich phases such as UAl₄, UMo₂Al₂₀ and U₆Mo₄Al₄₃ are produced.