Y2O3 films were grown on SiO2-covered Si(111), and hydrogen-terminated Si(111) substrates at a temperature of 500 degrees C by ultrahigh vacuum ionized cluster beam deposition. The microstructures and growth behavior of these films have been investigated by transmission electron diffraction (TED) and high resolution transmission electron microscopy (HREM). The TED results show that the Y2O3 grown On the SiO2-Si has the epitaxial relationship of (11-1)(Y2O3)//(111)(Si) and [-110](Y2O3)//[-110](Si). The film on the H-SI contains YSi2-x and amorphous YSixOy layers at the interface, having the orientation relationship each other. For the YSi2-x and the Si substrate, the relationship is (0001)YSi2-x//(111)Si and [1-210]YSi2-x//[-110]Si. For the Y2O3 and the YSi2-x, the relationship is as follows: (11-1)Y2O3//(0001)YSi2-x and [-110]Y2O3//[1-210]YSi2-x; (111)Y2O3//(0001)YSi2-x and [-110]Y2O3//[1-210]YSi2-x. The formation mechanism of the interfacial phases of SiOx, YSixOy and YSi2-x are described. It is shown that the crystallinity of the Y2O3 film on the SiO2-Si(111) is better than that of Y2O3 on H-Si(111).