The microstructures of 3,4,3',4'-biphenyltetracarboxylic dianhydride - p-phenylene diamine (BPDA-PPD) polyimide (PI) thin films with different thicknesses were investigated by grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XRR) at the SPring-8 synchrotron radiation facility. The film density calculated from XRR profiles increased with decreasing the film thickness. Processing of PI thin films from precursor polyamic acid (PAA) solutions involves simultaneous thermal imidization, evaporation of residual solvents and crystallization. The thickness reduction during imidization was smaller for the thinner PAA film. This implies that the much more larger amount of the residual solvent in the thinner PAA film brings high molecular mobility, which results in the higher conversion to PI at any curing temperature. The thinner BPDA-PPD film was more dense than the thicker film because of better molecular packing, which is a result of much more amount of the residual solvent for the thinner film during the thermal curing. (C) 2013 Elsevier Ltd. All rights reserved.