The surface of glass wool (GW) was modified by two different methods, either by grafting methyl acrylate (MA) onto GW in the presence of potassium persulfate (PPS) as initiator, or by adsorbing stearic acid (AS) as surface active agent onto the GW surface. It was found that conversion and grafting yield of poly(methyl acrylate) (PMA) onto the GW surface increased with increasing initiator concentration, whereas by modifying the GW surface by adsorption of SA, the adsorption of SA was found to be high at small values of equilibrium concentrations and two distinct plateau regions were observed. Mechanical and electrical properties of polypropylene (PP) sheets loaded with modified GW were measured. The obtained data showed that the presence of PMA as grafted polymer onto the GW surface has a plasticizing effect on the prepared PP composites. On the other hand, SA, as a surface modifier for GW, acts as a reinforcing agent for the formed PP composites, which have very good dielectric properties. From the obtained results, one can deduce that the prepared PP/GW composites have promising dielectric properties.