The characteristics of charge storage in APCVD Si3N4/thermal-grown SiO2 double layers electret films, charged by coroma were investigated by measurements of isothermal surface potential decay and thermally stimulated discharge(TSD).The results show that all of the samples have high charge stability at room temperature, at 300 and 60℃ under 95% of the relative humidity. The transport of the trapped charges due to external excitation for the negatively charged samples is basically controlled by the slow retrapping effect. However, the transport of detrapped positive charges due to external excitation can be described by a model of thermal ion emission.