The changes of atomic structure and the coverage of binary metals (Bi,Sb) at the Si(111) surface induced by thermal annealing have been studied by means of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and Rutherford backscattering spectrometry (RBS) techniques. It is shown that Bi and Sb form surface alloy of root 3 x root 3 structure. Moreover, coaxial impact-collision ion scattering spectroscopy (CAICISS) has been used to determine the atomic structures of the (Bi,Sb)/Si(111) surface. The structural model for the Si(111)root 3 x root 3-(Bi,Sb) surface is proposed on the basis of the polar- and azimuth-angle's dependence of the Si, Bi, and Sb intensities.