A fuel assembly comprises a plurality of fuel rods, tie plates for holding both ends of these fuel rods, and spacers which support these fuel rods. The spacer comprises a plurality of cells into which the fuel rods are inserted respectively, the adjacent cells being joined to each other at axial ends thereof, whereby a space between these cells being held or retained, and a plurality of loop springs held respectively on the cells. Each of the loop springs has a pair of resilient members which are located within the pair of adjacent cells and which urge the fuel rods in a radial direction, and a pair of connections which connect axial ends of the resilient members to each other. Each of the connections have a passage through which coolant flows axially and which is defined by a closed peripheral wall. The closed peripheral wall is not uniform in thickness. The pair of adjacent cells have at axial end portions of peripheral walls openings for accommodating or receiving the connections of the loop spring.