Ferroelectric films grown on Si substrates using Y2O3buffer layers were investigated by cross-sectional high-resolution transmission electron microscopy (HRTEM). It was found that no interfacial SiO2layer was formed when an Y2O3layer was grown by molecular beam epitaxy (MBE) on a Si(111) substrate at 600°C, while that a SiO2layer was formed on a Si(100) substrate under the same growth conditions. Then, ferroelectric YMnO3, SrBi2Ta2O9(SBT), and Pb(Zr,Ti)O3(PZT) films were deposited on the Y2O3/Si structures and cross-sections were observed. It was found that the interfacial SiO2layer about 5 to 6 nm in the thickness was formed even on a Si(111) substrate after deposition of the ferroelectric film. It was also found that polygrains in PZT film were much smaller than those in SBT, although both films were formed by the same sol-gel driven method.