A dependence of the soft magnetic properties of the Fe73.5Si13.5Nb3Cu1B9 alloy on the temperature of annealing (Tan) carried out in air for 2 hours at temperatures from 520 to 620°C was investigated. It was shown that with Tan increasing, the magnetic hysteresis loop broadens significantly and becomes more inclined, and the Curie temperature of the amorphous matrix surrounding the α-FeSi nanocrystals decreases. The atomic structure and phase composition of the alloy samples were investigated by X-ray diffraction in transmission geometry. After annealing at temperatures of up to 580°C, nanocrystals contain predominantly D03 phase (Fe3Si stoichiometry) and have average size of about 7 nm. Their relative fraction in the alloy increases as the temperature increases due to the additional diffusion of iron from the matrix into the nanocrystals. After annealing at Tan ≥ 600°C, the average size of the nanocrystals increases, and reflections of iron boride crystals appear in the diffractograms. The deterioration of the soft magnetic properties of the Fe73.5Si13.5Nb3Cu1B9 nanocrystalline alloy, when the annealing temperature rises from 520 to 580°C, is explained by a decrease in the silicon concentration in Fe-Si nanocrystals, which leads to a growth of the constant of the magnetocrystalline anisotropy.