Nanoparticles such as silica balls (monopheres), calcium carbonate, carbon nanotubes (CNT) or fibers (CF) are uniformly coated with a film of polyolefins of variable thickness using metallocene/methyl-aluminoxane (MAO) catalysts. The catalyst is adsorbed on the filler surface, leading to a polymer growth directly on its surface. Highly filled and stiff nano composites, basing on polyethene (PE) and syndiotactic or isotactic polypropene (sPP/iPP) with a content up to 85 wt% of silica or 7 wt% of multi-walled carbon nanotubes (MWCNT) are obtained. MWCNT treated with MAO in the same way form a heterogenous cocatalyst for the preparation of iso- and syndiotactic PP/MWCNT composites, too. The uniformity of the dispersion depends primarily on the pretreatment of the filler.