[en] Fuel assemblies constituting a reactor core contain fuel rods comprising mixed fuels of uranium and plutonium densely arranged so that an effective water-to-fuel volume ratio is less than 0.4. Two kinds of fuel assemblies having different weight ratios of fissionable plutonium relative to a heavy metal, that is, different enrichment degrees upon loading of fuel assemblies, are used. The enrichment degree of at least one of the fuel assemblies is 3 to 5 wt%. Then the number of fuel assemblies of low enrichment degree in an inner region is made greater than that of low enrichment degree in an outer region such that the number of fuel assemblies is identical between two regions when the reactor core is divided radially into two regions from the center. With such a constitution, a void coefficient can be improved (reduced) in the light water reactor of high conversion ratio without lowering the reactivity. (I.N.)