Solid bonding diffusion couples of U-Mo/Nb were made by clad and hot rolling. The diffusion couples were heat-treated at 790 °C for different time. The obversation of the diffusion layers was taken by scan electronic microscope and the main elements in different solid state phases were measured by energy dispersive spectrometer. The research results indicate that there exist were three different U-Nb phases coming into the Nb-base side of the diffusion couple heat-treated for 30 h. The phase components of the final solid state are associated with the phase transformation of U and the solubility of alloy elements in different U-phase