The charge stability has been systematically studied by measuring open-circuit thermally stimulated discharge (TSD) current spectra, in situ TSD charge spectra and isothermal charge decay for the polypropylene (PP) cellular films after chemical treatments ( extraction, oxidation and HF solution treatment). The results showed that a very strong new current peak, which the virginal film does not have, appeared at about 184℃ in the TSD current spectrum of the PP cellular film properly oxidized and then HF-treated at room temperature (RT), and that its charge storage thermal stability is significantly improved compared with that of the virginal PP film, and is enhanced further by high temperature charging method. The reason that charge stability of the HF-treated PP films is improved has been clarified by attenuated total reflection infrared (ATR-IR) spectroscopy. The charge stability of treated films can be improved by appropriately prolonging either HF treatment time at RT or oxidation time. The theory of in situ TSD charge indicates that an in situ TSD charge spectrum contains combined information on the changes of the mean charge depth of the deposited charge layer and the deposited charge value. Combining the TSD current spectrum and the initial mean charge depth of a PP cellular film at RT, the change of the mean charge depth can be in situ investigated in heating process.