A fuel rod position determination apparatus and a method are presented to enable efficient and accurate determination of the positions of the fuel rods in a fuel assembly in which it is not possible to insert a measuring instrument in a straight line through the fuel assembly. The apparatus first measures the positions of the peripheral fuel rods from the outside of the fuel assembly. This operation is continued by inserting sensors into the assembly through the spaces between the fuel rods so as to measure the profiles of the fuel rods. The positions of all the fuel rods including the interior fuel rods are determined on the basis of the profile data, the reference data formed by the peripheral fuel rods, and the known diameter data of the fuel rods.