Abstract: Neutron diffraction is performed on samples of the nanocrystalline soft magnetic material Fe 73.5 Cu 1Nb 3Si 13.5 B 9 which are anealed at three different temperature for 1 h respectively. Results reveal that amorphous ribbons of Fe 73.5 Cu 1Nb 3Si 13.5 B 9 are turned into a well ordering solid solution Fe 3Si(Fe) with DO 3 structure which is different from the Fe(Si) solid solution with bcc structure after they were annealed. And Si atoms and a few Fe atoms occupy the 4a site and Fe atoms mainly occupy the 4b and 8c sites, and the content of Si atom as well as the average grain dimension of nanocrystalline. Especialy, the reason that cell parameter enlarged is some B atoms go into the 48h site in 3 #(853 K) sample. It is obvious that 2 #(823 K) sample has the highest content of Si atom and the minimum average grain dimension(8.9 nm) comparing the results of the three samples. It seems the reason that the 2 #(823 K) sample has the best soft magnetism properties.