Long-wave infrared windows and domes used under harsh conditions always suffer sand abrasion and raindrop erosion. GaP thin film has proved to be effective LWIR protective coating for zinc sulphide window. In this article, the GaP films have been deposited on 5-mm-thick thermo-pressed planar ZnS substrates by RF magnetron sputtering in high pure Ar gas, with a single crystalline GaP disc as the target. The maxima of the GaP/ZnS transmission spectra curve are less than the corresponding transmissivities of ZnS substrates, which indicates that the GaP films are of absorption. SEM results show that the films are very compact and their surfaces are glazed. XPS analysis shows that gallium content is more than phosphor content in the films, and oxygen is the main impurity. The more the gallium content is, the more absorption the film is of. The absorption does not vary greatly with oxygen content. So the absorption of the film is caused mainly by the overfull metal component, which increases the conductivity and causes the charge carrier absorption. Low absorption GaP film with the thickness more than 10 µm has been deposited, in which the Ga:P ratio is nearly 1:1.