Electroless　Ni-Co　alloy　thin　films　on　fly-ash　hollow　microspheres　was　employed　to　study　it　electromagnetic　properties.　Metal　plating　was　carried　out　in　a　three　step　electroless　plating　process.　The　results　of　SEM　and　EDS　analysis　indicate　that　Ni-Co　could　be　plated　on　the　surface　of　cenospheres　comparatively　homogenous.　Microwave　absorbing　properties　were　determined　in　the　olefin　composites　containing　the　metal-coated　microspheres.　Due　to　the　conductive　behavior　of　Ni-Co　alloy　thin　films,　high　dielectric　can　be　obtained　in　microwave　frequencies.　A　number　of　experiments　show　that　electroless　Ni-Co　alloy　thin　films　on　fly-ash　hollow　microspheres　is　a　kind　of　microwave　absorbent.　With　the　increasing　of　the　electroless　Ni-Co　alloy　thin　films　on　fly-ash　hollow　microspheres　coat　thickness,　the　absorbing　peak　value　of　the　composite　shifts　to　lower　frequency.