In this work, melt-spun polypropylene (PP) fibers were treated in an electric field of a corona discharge. The fibers were then characterized using the thermally stimulated current (TSC) spectroscopy. It has been shown that the electret state of corona-treated PP fibers is a result of the combination of Maxwell-Wagner polarization and charge trapping. Activation energies and relaxation times for these processes have been determined, and characteristics of trapping sites have been calculated. The electret state induced in PP fibers by the corona discharge treatment holds for a long time (several months). Our analysis of the effect of processing temperature and electric field intensity on the characteristics of the electret state in melt-spun PP fibers allows one to specify optimum technological regimes for industrial production of PP-based electret: filter materials.