The amorphous $Fe_{73}Si_{16}B_7Nb_3Cu_1$ alloy strip was pulverized to get a flake-shaped powder after annealing at $425^{circ}C$ for 90 min and subsequently ground to obtain finer flake-shaped powder by using a ball mill. The powder was mixed with polyimide-based binder of $0.5{sim}3wt%$, and then the mixture was cold compacted to make a toroidal powder core. After crystallization treatment for 1 hour at $380{sim}600^{circ}C$, the powder was transformed from amorphous to nanocrystalline with the grain size of $10{sim}15nm$. Soft magnetic characteristics of the powder core was optimized at $550{sim}600^{circ}C$ with the insulating binder of 3wt%. As a result, the powder core showed the outstanding magnetic properties in terms of core loss and permeability, which were originated from the optimization of the grain size and distribution of the insulating binder.