The invention provides a heterogeneously loaded type of fast reactor core in which a reduced number of target fuel assemblies containing minor actinide nuclides are heterogeneously dispersed and loaded. A wrapper tube for each of the target fuel assemblies is of the same shape and size as a hexagonal wrapper tube for an ordinary core fuel assembly. Each of target fuel rods within the wrapper tube contains 20 to 50% of minor actinide nuclides, and has a diameter more reduced than that of an ordinary core fuel rod. The number of the ordinary core fuel rods loaded in the ordinary core fuel assembly is 271 while the number of the target fuel rods loaded in the target fuel assembly is 331 or 397. Thirty to 50 such target fuel assemblies are heterogeneously dispersed and loaded in the fast reactor core.