Abstract: The friction coefficient and wear resistance of a series of space liquid lubricants were investigated by the vacuum four-ball tribometer (VFBT) in air, vacuum (<6.0 × 10^-4 Pa) and high-purity nitrogen. The morphologies of the worn surfaces were observed using a scanning electron microscopy (SEM). The tribological tests in the air show that the selected oil exhibited lower and more stable friction coefficients with smaller wear scar diameters and smooth worn surface comparing with the ones obtained in vacuum and high-purity nitrogen. It is supposed that the forming of continuous boundary oxide film and the lubricant film was responsible for the different tribological behavior.