Fe73.5Cu1Nb3Si15.5B7 nanocrystalline powder cores with different particle sizes ranging from 10 to 125μm were fabricated by cold-pressing techniques.The cores exhibited increased core loss Pcvand decreased initial permeabilityμiwith addition of fine powders below 50μm in size,and the content should be less than 40mass%.It was thought to be closely related to the high coercive force Hcdue to the stresses generated during the crushing process and high demagnetization fields of small powders.Furthermore,modifying the alloy compositions by adding defined amount of Ni could improve the soft magnetic properties,including superior characteristics of permeability under high direct current(DC)bias field and comparable low core loss at high frequency. Fe73.5Cu1Nb3Si15.5B7 nanocrystalline powder cores with different particle sizes ranging from 10 to 125 μm were fabricated by cold-pressing techniques. These cores were increased core loss Pcvand decreased initial permeability μi with addition of fine powders below 50 μm in size, and the content should be less than 40mass% .It was thought to be closely related to the high coercive force Hcdue to the stresses generated during the crushing process and high demagnetization fields of small powders .Furthermore, modifying the alloy compositions by adding defined amount of Ni could improve the soft magnetic properties, including superior characteristics of permeability under high direct current (DC) bias field and comparable low core loss at high frequency.