Poling PVDP films with different crystal phases show that only the samples which are of β-phase and in direct contact with the poling electrodes can demonstrate strong piezoelectricity (E31*). The samples poled in contact with negative electrode during the poling process display stronger piezoelectricity (e31*) than the samples poled in contact with positive electrode. It is suggested that piezoelectricity of PVDF electrets arises from injected charges especially negative charges. During the poling process only PVDF samples which are of β-crystal phase can accept a large number of injected charges.