Abstract: Effects of simulated low earth orbit (LEO) atomic oxygen on tribological properties of two kinds of multiply-alkylated cyclopentanes (MACs) lubricants bearing with different lengths of branch chain are investigated by using a multi-function space tribo-system. The viscosity, mass loss, number-average molecule weights, chemical structure, and worn surface are characterized by viscometer, precision analysis balance, gel permeation chromatography, fourier transform infrared spectroscopy(FTIR) and scanning electron microscopy. The results show that the stability of mass loss of MACs after atomic oxygen can be efficiently improved by increasing the length of the branch chains. The two kinds of MACs have excellent lubricating properties for GCr15/GCr15 ball-on-disc friction pairs in high vacuum. The friction coefficient and the wear of the tested friction pairs increase after the MACs lubricants are irradiated by atomic oxygen. The FTIR results show that high energy atomic oxygen has inserted into the chemical bonds of MACs,and the new bonds including C=O and C—O—C are formed. This change results in the increment of the acid degree of MACs.