Abstract: In a fuel assembly for a pressurized water reactor and a method of designing the fuel assembly, fuel pellets made from a sintered uranium oxide are put into a zirconium metal alloy fuel cladding tube and form a fuel rod. The fuel rods are bundled together in a matrix form to form the fuel assembly. When an outer diameter of the fuel pellet increases, a weight of uranium loaded in the fuel assembly is increased. An amount of increase in reactivity caused by the decrease in burnup brought about by the increase in uranium weight exceeds an amount of decrease in reactivity caused by an insufficient moderation due to decrease in a moderator in a region A. The diameter of the fuel pellet is determined so as to fall within the region A where the increase in reactivity exceeds.