In this paper, series of polyimide films with different aggregation structures were prepared through a partially pre-imidization process, and the properties were investigated in detail. The pre-imidization degree (pre-ID) was successfully controlled by adjusting the amount of dehydrating reagents, which was confirmed by FT-IR spectra measurement. PI films exhibited a quite different degree of crystallinity, which increased with pre-ID increasing. DSC curves showed obvious melt endothermic peaks at about 450 degrees C. The tensile strength, tensile moduli and elongation at break of the PI films increased obviously with pre-ID increasing. The corresponding values of PI-100 increased by 37.9%, 11.7% and 336%, respectively, compared with those of PI-0. TMA measurement showed that the coefficient of thermal expansion of PI films decreased with pre-ID increasing, indicating that the PI molecular chains were restricted by the crystalline structure. Meanwhile, those PI films obtained by the pre-imidization process still possess excellent thermal stability. The present work provided the theoretic indicators for controllable preparing of high-performance polyimide films. (C) 2012 Elsevier Ltd. All rights reserved.