There are provided a fuel bundle for pressurized-water reactor and a method of designing a fuel bundle. In the forming of a fuel bundle through charging of fuel pellets obtained by burning/hardening of uranium oxide in a fuel cladding tube of zirconium alloy to thereby obtain fuel rods and bundling of multiple fuel rods in a lattice form, there is determined region A where the weight of uranium loaded in the fuel bundle is increased by an increase of the outside diameter of fuel pellets and where any increment of reactivity attributed to drop of combustion degree accompanying the same exceedsany drop of reactivity caused by insufficient moderation resulting from a decrease of moderating material due to the increase of the outside diameter of fuel pellets, and the diameter of fuel pelletsis established within the reactivity increase region A.