The surface Debye-temperature of the topmost surface of the Si(1 1 1)-7 x 7 has been determined by reflection high-energy positron diffraction. The positron diffraction intensity at the total reflection condition is altered by the thermal vibration amplitude of atoms only on the topmost surface because the incident positrons are not able to penetrate into the bulk in this condition. The intensity of totally reflected positrons was analyzed by means of the dynamical diffraction theory. The surface Debye-temperature was determined to be 310 +/- 50 K, which is smaller than those determined in previous studies using electron diffraction. The vibration amplitude was estimated to be 0.13 Angstrom at 293 K. (C) 2004 Elsevier B.V. All rights reserved.