In connection with the influences of the densification of sample, the evaporation of diffusant during diffusion annealing and the grain size of starting material on the diffusion of actinide elements in UO_2, three types of experiment were performed by the sectioning method using ^<237>U and ^<233>Pa as tracers. In all cases, the logarithm of the concentration in a deeper region varied linearly with penetration depth, as is characteristic of grain-boundary diffusion. Near the surface, the lattice diffusion or the evaporation made a considerable contribution to the concentration, thus showing a nonlinear concentration profile. The extent of this deviation increased with the increases of initial density of the pellet and of grain size of starting material. The concentration near the surface decreased with the approach to it in the presence of evaporation.