Piezoelectrets are a kind of space-charged electret material with a void-structure. The piezoelectric effect in such a material is related with its microstructure and space charge. In this paper the micro-structure of polypropylene (PP) films is first modified by using a pressed gas expansion process to enhance the charging capability of the films, and then a direct contact charging is carried out to polarize the expanded films. The relationship between the applied voltage and the space charge density, and the influence of the applied voltage on the piezoelectric performance of PP films are investigated. Results show that for 100 m thick modified PP films, the critical voltage necessary for the build-up of the macro-dipoles in the inner voids is approximately 2 kV; once the macro-dipoles are built up, the PP films will exhibit piezoelectric effect. With increasing polarization voltage, the space charge density gradually increases, resulting in significant enhancement of piezoelectric effect. For the PP films polarized at a peak voltage of 8 kV, the space charge density, d33 coefficient, and the figure of merit FOMv(d33 g33) are 0.56 mC/m2, 379 pC/N and 8.6 GPa-1, respectively. Since not only the FOMv of the PP films is almost two orders of magnitude larger than that of PVDF, but also the acoustic impedance in such a material is very low (～ 0.025 MRayl), the PP films have an obvious advantage as applied in airborne ultrasonic transmit-receive or pulse-echo systems.