Abstract: According to the value of energy difference between Ga3d core level and the valence band maximum of GaAs precisely determined by Kraut et al., the position of Fermi level with respect to the valence band maximum at the surface and the surface band bending of clean GaAs (111) can be determined by measuring the exact binding energy of Ga3d level in the UPS experiment. The electron affinity of GaAs can also be obtained by measuring the work function from the threshold of secondary electron energy distribution in. the UPS spectrum. For the clean GaAs (111) (2×2) Ga-riohed surfaces prepared by Ar ion sputtering followed by heat annealing, the Fermi level pinning at the surface can be found with a pinning level sited about 0.75 eV above the valence band maximum, which coincides with the acceptor level observed by Spicer's group on. GaAs (110) surface.