A chlorination surface treatment was used to reduce the surface density of states of a n-type GaN surface, which improves the Schottky performances of the resultant metal-semiconductor contact. Using capacitance-frequency measurement, the surface state density of the chlorine-treated GaN surface was about one order less than that without chlorination treatment. The dark current of the chlorine-treated GaN ultraviolet photodetectors (UV-PDs) is 1.5 orders of magnitude lower than that of those without chlorination treatment. The products of quantum efficiency and internal gain of the GaN Schottky UV-PDs without and with chlorination treatment under conditions of -10 V reverse bias voltage at a wavelength of 330 nm were 650% and 100%, respectively. The internal gain in chlorine-treated GaN UV-PDs can therefore be reduced due to a decrease in the surface state density.