[en] A fuel assembly in which fuel rods are arranged in a square lattice and at least one neutron moderating rod having a moderator cross sectional area greater than that of a unit fuel lattice is contained, fuel rods comprise a plurality of first fuel rods and second fuel rods each having an effective fuel length shorter than that of the first fuel rods. The second fuel rods are arranged at other portions than the corners of the outermost layer of the square lattice-like fuel rod arrangement. Among fuel rods arranged at a layer in adjacent with the outermost layer of the fuel rod arrangement, first fuel rods are arranged in adjacent with the second fuel rods. When moderators in non-boiling water region are arranged locally in a fuel assembly in which the number of lattice rows is increased to more than 9 x 9, a greater effect can be obtained for improving the controllability of reactivity by localizing the moderators to the outer region of the fuel assembly facing a channel box than to the inner region of the fuel assembly facing a water rod. The effect of reducing a void coefficient is thus obtained to improve the controllability of reactivity, an effect of improving control worth can be expected thereby contributing to the improvement of safety. (N.H.)