The trap structure of polyimide was studied using open-circuit thermally stimulated discharge current (TSDC). TSDC was observed in 25-mu m thin film of polyimide at different condition of polarizing field and temperature. The one-sided vacuum-aluminized samples were used to record the open-circuit thermally stimulated current. The dipolar mechanism of relaxation is a possible mechanism to explain the behavior of TSDC peak at low temperature and field condition; however, hopping transport mechanism is prominent for the sample polarized with higher field and temperature. This is because deeper traps capture the charge carriers at high polarizing field and temperature and their subsequent trapping through hopping process. The influence of surface traps is caused by thermal polarization; it is evident due to lower value of activation energy, since activation energy of TSDC peak is corresponding to surface traps. The morphology of surface traps is verified by measuring the surface roughness using atomic force microscopy. The both type of charges (i.e., heterocharge and homocharge) are responsible for flow of current in presence of air gap. We report the role of surface and deep traps induced due to charging process. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012