Abstract: B was substituted by Si and Co on basis of the composition of Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy, and then Fe_73.5Co_0.3Cu₁Nb₃Si_14.2B₈ alloy strips were prepared by single roll fast quenching method. The tested alloy strips were winded into toroidal magnetic cores, and then these cores were annealed at different temperatures. The influence of Co addition on the crystallization behavior of alloy strips and soft magnetic properties of magnetic cores were studied. The results show that Co addition had little effect on crystallization behavior of alloy strips. The tested alloy strips had an amorphous structure, and the magnetic cores after annealing at 550℃ had the co-existence of amorphous and nanocrystalline structures. After addition with Co, the initial permeability and saturation induction density of tested alloy amorphous/nanocrystalline magnetic cores were reduced by 0.149 9 H·m^-1 and 388 T, respectively, while the coercivity increased by 0.338 5 A·m^-1, indicating the direct-current soft magnetic properties decreased. The effective amplitude permeability, inductance and quality factor at high frequencies (less than 100 kHz) of tested alloy amorphous/nanocrystalline magnetic cores increased while the total loss density and coercivity decreased, indicating the alternating-current soft magnetic properties were improved.