The atomic structure of (NH₄)₂ Sx-treated and annealed GaAs surface has been investigated using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). STM observation revealed that most of the GaAs flat surfaces showing 2×1 structure are terminated with Ga-S bonds and that the sulfur monolayer forms dimers after being annealed at 510°C. Surface structural parameters such as S-S dimer bond length and Ga-S distance in the [100] direction are determined and found to be in good agreement with the theoretically calculated values. A sulfur overlayer is also observed. The second S layer has a 2x structure in both [100] and [110] directions, but the third layer does not show any ordered structures. AFM observation shows that the S-treated and annealed GaAs is covered with a thin S layer in a wide area and that many circular S residuals reside on the surface. These results reveal that the surface is covered not only with an S monolayer but also with an S multilayer even with high-temperature annealing at 510°C.