The Cu content dependence of magnetic properties in annealed (Fe_0.85B_0.15)_100−xCu_x alloys fabricated by melt-spinning was discussed. The value of H_c markedly decreases with increasing x between x=1.0 and 1.5, accompanied with significant reduction of crystalline grain size. The alloy with x=1.5 showed excellent magnetic properties such as a small H_c of about 7 A/m and a high B_s of more than 1.8 T. For the present alloy system, more than 1.0% addition of Cu is effective for the formation of nano-scale grains and for the improvement of the soft magnetic properties.