Polycrystalline Fe78B13Si9 alloys with nanoscale grain sizes prepared by the crystallization method have been studied by positron lifetime and Doppler‐broadened line‐shape measurements. The results obtained are different from those on amorphous or coarse‐grained crystalline alloys with the same composition. When the grain sizes are clearly smaller than the mean positron diffusion length (L+≂100 nm), saturation trapping and annihilation of the positrons in the samples should occur at traps within the interfaces. There are two kinds of defects in the interfaces of the nanocrystalline FeBSi alloys, i.e., the free volumes (the size of which is slightly smaller than that of the amorphous counterpart) and the microvoids. The dependence of grain sizes on the type of interface defects and structure‐sensitive properties has also been studied by positron spectroscopy. The measurements and comparison of the mean positron lifetime τ¯ and the Doppler parameter S on the nanocrystalline, amorphous, and coarse‐grained crystalline alloys have given a satisfactory explanation for the relationship between the structure‐sensitive properties, grain sizes, and crystalline states.