Reflection high-energy positron diffraction (RHEPD) patterns from the Si(l 1 1) 7 x 7 clean surfaces were observed at glancing angles <1.4degrees, where the positron total reflection occurs, using a well-collimated positron beam with an energy of 20 keV Fractional order diffraction patterns assigned to the 1/7th, 2/7th and 3/7th Laue zones were confirmed. RHEPD patterns were analyzed based on kinematical and dynamical diffraction theories. Although the intensity profile calculated from the kinematical theory qualitatively reproduced our experimental results, dynamical calculations were needed for the quantitative discussion. The effective penetration depth of the incident positrons in the total reflection region were 1-2 Angstrom, significantly shallower to that observed in electron diffraction. RHEPD allows a more precise investigation of topmost surfaces than electron diffraction. (C) 2004 Elsevier B.V. All rights reserved.