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