The thermal curing (imidization) of the polyimide film has been investigated using DSC and FTIR spectroscopy. The rate and the degree of imidization of the film was found to be dependent on curing temperature, time and thickness. The degree of imidization of the film rapidly increased between 200°C and 350°C and then remained constant for curing above 350°C, indicating that the imidization reaction is complete at 350°C. Also, it was found that imidization proceeds in two stages under isothermal condition, which was characterized by an initial fast imidization step that changes into a second, slower imidization process. The slower imidization rate at the second stage is discussed and explained. Higher degree of imidization was obtained with thicker films because of the higher retention of the solvent in the thicker film. Depolymerization process during thermal curing has also been studied with FTIR. The highest degree of the depolymerization is reached with thermal curing between 225 and 275°C.