The modified conversion ratio (MCR) (the ratio of the 238U capture rate to the total fission rate) in a light-water-moderated uranium-plutonium mixed-oxide- (MOX-) fuel lattice was measured for four types of lattices with different plutonium enrichment. In the current method, the relative reaction rates of 238U capture and total fission were obtained from nondestructive gamma-ray spectrometry of 239Np and fission products, respectively, which accumulated in the fuel rod irradiated at the Tank-Type Critical Assembly. The measured results of the fission rates derived from two different fission products agreed well with each other, and the measured MCRs showed good agreement with the results of the Monte Carlo calculation with the whole-core model. Therefore, the current nondestructive method is applicable to the MCR measurement of MOX fuel.