Strain behavior of polycrystalline Nb thin films deposited onto oxidized silicon substrates using an electron gun in a high vacuum system was studied by an x-ray diffraction technique. The sign and magnitude of strain in Nb thin films were found to have a strong dependence on the film thickness. For films thinner than approximately 10 nm, the compressive strain was observed, which was considered to be due to diffusion of impurity atoms, most likely oxygen, into grain boundaries of Nb films within an extremely short distance from the film surface. For thicker films, the tensile strain was observed and a strong dependence of the strain (normal to the film surface) on crystal orientations of grains was observed. This dependence agreed very well with the one calculated using a biaxial stress model. The biaxial stress applied in the planar direction was calculated to be 1.0×1010 dyne/cm2 from the measured strain values. The present result indicates that the strain in the Nb films is not induced by temperature increase during the film preparation, but it is more likely due to grain growth during and/or immediately after the film deposition. Based on the biaxial stress model and strain distributions parallel and normal to the substrate planes and strain energies stored in grains with different crystal orientations were calculated.