The ultraviolet photoelectron spectroscopy of GaAs (100) surface (4×1) structure has been measured by using a polarized UV light source. From the differential spectra between the UPS of the clean surface and the surface with adsorbed oxygen, the surface state peak located within 2eV below valence band maximum could be identified. According to the spectra obtained with s- and p-polarized light and the selection rule of electron transition, it could be shown that this surface state cosists of three peaks: the peak at 0.5eV below valence band maximum corresponding to the bridge bond states of surface Ga atoms, the peak at 0.7eV resulting from the bridge bond states of surface As atoms, and the peak at 1.3eV related to the dangling bond states.