In the paper, the studies of electret properties of polypropylene with different percentages of montmorillonite have been carried out by methods of thermally stimulated potential relaxation and thermally stimulated currents. A significant effect of filler concentration on the electret state stability was revealed. The parameters of electrically active defects and the storage time of the electret state were determined. It was established that polypropylene with a 4% mass content of montmorillonite exhibited the best electret properties.