To investigate the accuracy of the neutronic calculations in various neutron spectra, the modified conversion ratios [(MCR): ratio of 238U capture rate-to-total fission rate] of four kinds of light water-moderated UO2 fuel lattices were measured. In the measurements, the relative reaction rates of 238U capture and total fission were obtained from the nondestructive gamma-ray spectrometry of 239Np and 143Ce, respectively, which accumulated in the fuel rod irradiated at the Tank-Type Critical Assembly. The moderator-to-fuel volume ratios Vm/Vf of the measured lattices were 1.50 (undermoderate) to 3.00 (overmoderate). The measured MCRs were 0.477 ± 0.014(Vm/Vf = 1.50), 0.434 ± 0.013(1.83), 0.383 ± 0.011(2.48), and 0.356 ± 0.011(3.00), respectively. The Monte Carlo calculation employing the JENDL-3 library showed good agreement with the experiments for all the cores, although they showed a tendency of overestimation for larger values of MCR, as shown in the case of UO2 tight lattices. Therefore, it was concluded that, for the cores investigated, the accuracy of the neutronic calculation method currently used was very good.