The piezoelectrets made of porous polytetrafluoroethylene （PTFE） films and nonporous fluoroethylenepropylene （FEP） films are prepared by using a hot-pressing method. The quasi-static piezoelectric d33-coefficients of such piezoelectret films are measured. The thermal stability of d33-coefficients and dynamic characteristics of charges in the piezoelectret films are investigated. The dynamic d33-coefficients are obtained by analyzing the dielectric resonance spectra of the piezoelectret films. The results show that the piezoelectric d33-coefficients up to 300 pC/N are achieved for the laminated PTFE/FEP films. The d33 value retains 40% of the initial value when the sample was annealed at the temperature of 90℃ for 20 h. Pre-ageing is an effective way to further improve the thermal stability of d33-coefficients. The dominant drift path of the detrapped charges in the shallow traps is most likely along the surface of the PTFE fibers, while charge drift through the solid layer of FEP is possibly prevailing for the charges in the deeper traps. The d33 value determined from the dielectric resonance spectrum is smaller than the quasi-static d33 value, which is mainly due to the enhanced Young's modulus with the increase of frequency.