A chlorination surface treatment was used to reduce the surface states of an n-type GaN surface, which improves the Schottky performances of the resultant metal-semiconductor contact. At a reverse bias of 10V, the dark current of the GaN-based UV-PDs with and without chlorinated surface treated were 28.1nA and 0.59μA, respectively. The dark current of chlorine-treated Schottky UV-PDs was 21 times of magnitude smaller than that of those without chlorination treatment. The product of quantum efficiency and internal gain of the GaN Schottky UV-PDs without and with chlorination treatment under a reverse voltage of 10V at a wavelength of 330nm was 650% and 100%, respectively. The internal gain of chlorine-treated GaN UV-PDs can be reduced due to the improvement of surface state density.