There is provided a novel initial core arrangement of a nuclear reactor. The fuel assemblies in the central region of the reactor core occupying more than 50% of the total area of the reactor core are grouped into a plurality of fuel units each of which being constituted by four adjacent fuel assemblies. The fuel assemblies in each fuel unit are replaced one by one at each fuel replacement cycle with a new fuel assembly. The initial core is so arranged that each fuel unit has one gadolinia containing high enrichment fuel assembly and three low enrichment fuel assemblies containing no gadolinea. This arrangement permits a stable reactor operation substantially equivalent to that obtained in the state of equilibrium (equilibrium core), even with an initial core.