Reflection high energy electron diffraction (RHEED) patterns from the Si(111)√21 × √21-(Ag,Au) surface are analyzed by kinematical calculations. The models including three, four, and five gold atoms in a unit cell are examined for the structural analysis. The best model is concluded that the surface atomic structure includes three gold atoms in the unit cell. [DOI: 10.1380/ejssnt.2008.84]