The microstructure and soft magnetic properties of Fe83Si3B11P2Cu1 (at.%) nanocrystalline ribbons with different thicknesses up to 40 μm obtained by annealing the amorphous/nanocrystalline precursors at various heating rates (HRs) were studied. It was found that the thicker melt-spun alloy ribbons contain more and coarser α-Fe grains owing to the lower cooling rates during the solidification of melt spinning. By rapidly annealing the relatively thick amorphous/nanocrystalline precursors with the large-size pre-existing α-Fe grains at a high HR of about 100 °C/s, the resultant nanocrystalline alloys exhibit low coercivity (Hc) of about 5.2 A/m and high saturation magnetization (Bs) of about 1.83 T, which are approximate to the nanocrystalline alloys acquired by annealing the precursors with an amorphous structure. The strong competition among the formation of primary nuclei and the growth of the pre-existing and the newly-nucleated α-Fe induced by the more rapid annealing could effectively suppress the coarsening of the pre-existing α-Fe with the large size up to about 30 nm, which facilitates the formation of the fine and uniform nanostructure and magnetic softening of the resultant nanocrystalline alloys.