High-quality single-crystal nanothick Y2O3 films have been grown epitaxially on Si (111) despite a lattice mismatch of 2.4%. The films were electron beam evaporated from pure compacted powder Y2O3 target in ultrahigh vacuum. Y2O3 3 nm thick exhibited a bright, sharp, streaky reconstructed (4x4) reflection high energy electron diffraction pattern. Structural studies carried out by x-ray diffraction with synchrotron radiation and high-resolution transmission electron microscopy show that the films have the cubic bixbyite phase with a remarkably uniform thickness and high structural perfection. Two Y2O3 domains of B-type Y2O3 [2 (1) over bar(1) over bar]parallel to Si[11 (2) over bar] and A-type Y2O3 [2 (1) over bar(1) over bar]parallel to Si[2 (1) over bar(1) over bar] coexist in the initial film growth with B type predominating over A type in thicker films as studied using x-ray diffraction. The narrow full width at half maximum of 0.014 degrees in the omega-rocking curve is the characteristic of excellent crystalline films. High-resolution transmission electron microscopy and fast Fourier transform analysis show atomically sharp interface and strain relaxation in thicker films. (C) 2008 American Vacuum Society.