A fuel assembly suitable for use in boiling water reactor. The fuel assembly has a specific pattern of distribution of enrichment, so that the upper region of the assembly has a larger infinite multiplication factor than that of the lower region. In an example, the distribution of enrichment is such that the upper region has a mean enrichment larger than that of the lower region, while, in another example, the mean enrichment is maintained constant over the length of the assembly but the difference of enrichments between the central and peripheral portions of a plane normal to the axis of the assembly is made smaller at the upper region than at the lower region.