A fuel assembly is composed of first fuel rods containing nuclear fuel material and burnable poison material and second fuel rods containing nuclear fuel material but no burnable poison material. The fuel assembly is charged in a high conversion area of a boiling water type nuclear reactor having a reactor core having the high conversion area and a burner area surrounding the high conversion area. A ratio V.sub.C /V.sub.F is not greater than 1.5 in the fuel assembly where V.sub.C is the volume occupied by coolant flow paths and V.sub.F is the volume occupied by the nuclear fuel material per unit length in an axial direction of the fuel assembly. In addition, the burnable poison material is composed of neutron absorption nuclides having at least one resonant energy in a neutron energy region of one electron volt or less.