The double heterogeneity effect due to the fuel pin and the subassembly is estimated for neutronics parameters of a prototype fast power reactor. Both of the heterogeneity effects caused by the flux fine structure and the resonance shielding are taken into account. The model of the hexagonal unit subassembly consists of the smeared fuel, the wrapper tube, and the outer sodium regions, where the average cross sections of the smeared fuel region are obtained by a unit pin cell calculation in cylindrical geometry. It is shown that the equivalent smeared model agrees well with the refined cluster model. A formula to define the background cross section of the fuel isotopes is derived based on the rational approximation, and its accuracy is examined. This formula can take into account the contribution from the wrapper tube to the background cross sections of each ring of fuel pin array. The heterogeneity effect of the whole reactor model is calculated based on two-dimensional diffusion theory and perturbation theory. The double heterogeneity effect is found to be 0.5%Δk for keff, the positive sodium-void worth is reduced by 26%, and the negative Doppler reactivity increases by 7% for a prototype fast breeder reactor. These results are considerably larger than the estimates made by earlier workers. These effects are significant from the viewpoint of target accuracy goals in fast power reactor design.