Phase transition of Si(111)-√3×√3-Ag surface has been investigated in the temperature range from 103 K to 293 K using reflection high-energy positron diffraction. Temperature dependences of the diffraction spot intensities below the critical temperature (T_c=116 K) are well explained in terms of the disordering of the inequivalent triangle (IET) structure. From the structure analyses of the rocking curves obtained above and below T_c based on the dynamical diffraction theory, it is concluded that the order-disorder phase transition takes place. [DOI: 10.1380/ejssnt.2005.228]