Provided is an apparatus for measuring outer diameters of fuel rods of a nuclear fuel assembly, in which the fuel rods are standing upright and are used in a light water reactor. The apparatus includes a support unit fixed to a floor at a lower portion thereof so as to be kept insulated from vibrations caused by external forces, a measuring unit mounted on the support unit and moving up and down to measure the outer diameters of the fuel rods, and a transducer cooperating with the measuring unit, converting the measured outer diameters of the fuel rods into electrical signals, and sending the converted signals to the outside. Thus, the apparatus moves up and down to measure the outer diameters of the fuel rods, so that it can accurately check abnormalities in the outer diameters of the fuel rods.