We examined a method for coating zirconia on flat amorphous Co70.3Fe4.7Si10B15 powder (fa-Co) by atmospheric pressure glow plasma with Zr(OC4H9)(4). XPS analysis was performed to determine the existence of ZrO2 on the surface of the treated powder. The fa-Co obtained strong resistance to oxidization by this zirconia coating method. Resistivity measurements revealed that fa-Co coated with ZrO2 was insulating. We also improved the permeability as a function of frequency of composite cores made from fa-Co by this zirconia-coating method.