Nickel powders coated by cobalt (Co@Ni powders) on the surface have been obtained by reduction in the ethylene glycol. The morphology and composition were characterized by scanning electron microscope and energy dispersive X-ray spectroscopy. The valence states of cobalt in the sample were determined by X-ray photoelectron spectroscopy. The Co@Ni powders were used as the electrically conductive fillers in the polymeric positive temperature coefficient (PPTC) materials in which polyethylene was used as the matrix. The stability of the resistivity of PPTC materials has been improved due to the protection and electrical conductivity of the cobalt layers on the surface of Ni powders. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012