The article is devoted to the study of the effect of the carbon black on the stability of the electret state in the plates of LDPE, charged in the corona discharge at room temperature. The kinetics of charge relaxation in LDPE plates under isothermal conditions in a wide temperature range was studied. The dependence Ichf(t) in the temperature range 70–90 °C was obtained. It is shown that the relaxation of the charge is not due to intrinsic conductivity, but is caused by the release of carriers from traps.