We have observed the Si(1 1 1)-root 3 x root 3-Ag surface using reflection high-energy positron diffraction (RHEPD). The RHEPD pattern clearly displays the intense (+/- 2/3 +/- 2/3) spots, which results from the large magnitude of the scattering factor for the Ag atoms. The rocking curves of the RHPED at room temperature have been measured and analyzed by means of the dynamical diffraction theory. We found that the atomic height of the topmost Ag triangle is 0.77 angstrom and is well in accordance with those determined by the other methods. (c) 2004 Elsevier B.V. All rights reserved.