Tin oxide (SnO2) nanolayers were formed on flexible polyimide (PI) substrate via direct ion-exchange and in situ oxidation process utilizing pyromellitic dianhydride/4,4′-oxidianiline-based poly(amic acid) films as polyimide precursor. During an ion-exchange process, stannous ions were doped into the precursor by immersion in ethanolic solution of stannous chloride. Subsequent thermal treatment of the tin(II)-containing precursor at a constant heating rate not only imidized poly(amic acid) to PI but also converted stannous ions into SnO2 clusters, which diffused and aggregated onto the surface of polymer matrix, forming continuous tin oxide layers. Inductively coupled plasma (ICP) was used to investigate the ion-exchange process. Changes in chemical structure of the poly(amic acid) film and the crystal structure of tin oxides were analyzed by attenuated total reflection-Fourier transform infrared (ATR-FTIR) and X-ray diffraction (XRD). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the microstructure of the PI/SnO2 nanocomposite films. The nanocomposite film maintained essential mechanical property and thermal stability of pristine PI films.Keywords: direct ion exchange; in situ oxidation; nanocomposite film; polyimide; tin oxide