Abstract: The friction coefficient and wear resistance of two kinds of space liquid lubricants named perfluoropolyether (Z25) and chlorinated-phenyl and trifluorinated-butyl with methyl terminated silicone oil (FCPSO) were investigated by the vacuum four-ball tribometer (VFBT) in air, vacuum and high-purity nitrogen. The morphologies and element analysis of the worn surfaces were observed using scanning electron microscopy and x-ray photoelectron spectroscopy, respectively. The results of the tribological tests in the three kinds of atmospheres all show that, due to both the lubricating and corrosive effects of Z25 to steel, it exhibited lower friction coefficients with larger wear scar diameters. Comparing Z25, FCPSO exhibited higher friction coefficients with smaller wear scar diameters, which was mainly because of the anti-wear boundary film formed by FCPSO.