The resonance self-shielding effect of fission products on burnup characteristics has been investigated in high conversion light water reactors. Reactivity loss by burnup was considerably reduced by taking account of the self-shielding effects of fission products. The effect caused a difference of ∼0.6% for the multiplication factor at 50 GWd/tonne burnup and it contributed to a negative void reactivity. Furthermore, the mutual shielding effects of resonance overlapping among actinides and fission products have been examined and observed for several fission products. The effect of nuclear data uncertainties of fission products on the burnup reactivity change has been also examined by comparing the results obtained with four evaluated nuclear data files: JENDL-2, JEF-1, ENDF/B-IV, and -V. Fractional absorption rates for individual fission product nuclides were considerably scattered among these files. A significant difference between the reactivity changes calculated with JENDL-2 and ENDF/B-V was observed, while the discrepancy between those obtained with JENDL-2 and JEF-1 was small due to an accidental cancellation.