Neutron streaming in a fast breeder reactor fuel subassembly caused by the double heterogeneity of the pin structure and the wrapper tube structure is estimated by double heterogeneous modeling. The neutron streaming is decomposed into three components: the pin-cell heterogeneity, the wrapper tube heterogeneity, and the homogenized fuel/wrapper tube subassembly effect. The streaming effect is evaluated based on the Benoist diffusion coefficient. The total streaming effect caused by the double heterogeneity structure of a fuel subassembly is found to be about −0.2% Δk/kk′ for keff, which is almost twice that obtained from the conventional pin-cell model of about −0.1% Δk/kk′.