Abstract: Solid bonding diffusion couple of U-Mo/Nb was prepared by covering and hot rolling. The diffusion couple was heat-treated at 760 ℃ for different time (3-15 h) and different thickness diffusion layers were gained. In order to match the relationship of heat-treated time with diffusion layer thickness, line fitting was performed. The result shows that the growth of diffusion layer has a process for gestation in the first instance. The diffusion layer was observed by means of SEM and EDS. The diffusion phenomenon of crackle at the side of U-Mo alloy after long-time heat-treatment is primary studied and discussed. The element in diffusion couple after heat-treatment was analyzed. The results indicate that the thickness increase of the diffusion layer is just the U element homogenize in Nb baseplate after long-time heat-treatment.