The influence of the oxygen environment on the transport behavior of a cadmium sulfide (CdS) single crystal nanobelt is investigated by device performance under various light illuminations and oxygen partial pressures. The CdS nanobelt shows superior photo response in the visible light region and the conductance is sensitive to the oxygen environment. The results show that when exposed to oxygen, the surface chemisorbed oxygen species and their interactions with surface defects will significantly affect the conductivity by decreasing the carrier concentrations and reducing the mobility. The effect is explained by surface band bending which is observed by surface potential measurement. Furthermore, we show that the height of the nanobelt is one of the critical factors that greatly affects the conductance due to the intrinsic belt-like geometry.