The water atomization Fe_73.5Cu₁Nb₃Si_15.5B₇ nanocrystalline magnetic powder cores with better high frequency magnetic properties were prepared. The effect of ball milling on magnetic properties of it was researched. Such results were concluded from experiments. With increasing the milling time, the powder size decreased and core loss increased. The effect of milling to permeability was not obvious. With milling 2 hours magnetic properties of powder core were best enhanced.