Abstract: The properties and microstructure of Fe_73.5Nb₃Cu₁Si_13.5B₉ new ultrafine crystalline soft magnetic alloy have been investigated.The results have been shown that the alloy is combined with high relative initial permeability of μ_(0.08A·m)=14.8×10⁴. low coercivity of Hc=0.6A/m. high saturation magnetic flux density of B_800A·m,=1.22 T under the optimum performance with annealing at 535℃, the optimum values of core loss have reached P_5·10k=5.6W/kg, P_5·20k=17.4W/kg, P_5·50k=80W/kg. P_5·100k=270W/kg. The alloy has high effective permeability μ'₁ in a considerable wide frequency range. The microstructural analysis under X-ray diffraction was detemined that the main crystalline phase in the alloy is xFe-Si solid solution which has ultrafine grain size of d=1 1nm. lattice parameter of a a₀=0.2 837nm.