A fuel assembly according to the present invention is formed in such a manner that the small diameter portion of an upper end plug of a fuel rod is inserted into a boss of an upper tie-plate. The upper end plug includes a small diameter portion and a large diameter portion which is disposed below the former. An expansion spring in disposed in the small diameter portion of the upper end plug. A cover member surrounding this expansion spring is disposed in the large diameter portion of the upper end plug, and extends upwardly. Since the cover member surrounds the expansion spring, pressure loss caused by the expansion spring can be reduced.
