Self-shielding factors for inelastic scattering cross sections of iron are calculated from the evaluated nuclear data flies ENDF/B-IV and JENDL-2. The effect of self-shielding by inelastic scattering on the neutron spectra is studied for fast reactor assemblies ZPR-3-54 and MZB, the fast critical assembly of the Japanese fast reactor program. The effect is very pronounced, especially in the steel reflector, where the spectrum is shifted to higher energy. The radial distribution of fission reaction rates calculated by considering that the self-shielding effect becomes smaller for 235U and 239Pu, but larger for 238U in the blanket and reflector regions.