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Structure,crystallization behavior,and magnetic properties of as-quenched and annealed Fe81.3Si4B13Cu1.7 (Cu1.7) alloy ribbons and effects of Nb alloying have been studied.Three-dimensional atom probe and transmission electron microscopy analyses reveal that high-number-density Cu-clusters and Pre-existing Nano-sized α-Fe Particles (PN-α-Fe) are coexistence in the melt-spun Cu1.7 amorphous matrix,and the PN-α-Fe form by manners of one-direction adjoining and enveloping the Cu-clusters.Two-step crystallization behavior associated with growth of the PN-α-Fe and subsequent nucleation and growth of newly-formed α-Fe is found in the primary crystallization stage of the Cu1.7 alloy.The number densities of the Cu-clusters and PN-α-Fe in melt-spun Fe81.3-xSi4B13Cu1.7Nbx alloys are gradually reduced with enriching of Nb,and a fully amorphous structure forms at 4 at.％ Nb,although smaller Cu-clusters still exist.After annealing,2 at.％ Nb coarsens the average size (Dα-Fe) of the α-Fe grains from 14.0 nm of the Nb-free alloy to 21.6 nm,and 4 at.％ Nb refines the Dα-Fe to 8.9 nm.The mechanisms of the α-Fe nucleation and growth during quenching and annealing for the alloys with large quantities of PN-α-Fe as well as after Nb alloying have been discussed,and an annealing-induced α-Fe growth mechanism in term of the barrier co-contributed by competitive growth among the PN-α-Fe and diffusion-suppression effect of Nb atoms has been proposed.A coercivity (Hc) ∝ Dα-Fe3 correlation has been found for the nanocrystalline alloys,and the permeability is inverse with the Hc.