In the present study, we investigated the effect of heat treatment on the magnetic properties of the Fe87-xSixB9Nb3Cu1 (x = 1, 3, 5, 7, 9, and 13.5) alloy system. Ribbon samples with the width of 2 mm and thickness of 20-30 mu m were synthesized by melt spinning. Among the samples, only amorphous samples were nanocrystallized by annealing between 360 and 600 degrees C. The annealed alloys tended to have improved soft magnetic characteristics, including higher saturation magnetic flux density (B-s), while maintaining high relative permeability (mu(r)), and low core loss (P-cv). A high saturation magnetic flux density of 1.51 T was obtained for the Fe78Si9B9Nb3Cu1 core sample after longitudinal magnetic field annealing at 520 degrees C. The sample had excellent soft magnetic properties as compared to the conventional value of FINEMET core (B-s = 1.23 T), which is a significant improvement as compared to the as-spun ribbons (B-s = 1.10-1.26 T).