The inclusion complex of p-cyclodextrin (beta-CD) and dialkyl pentasulfide (DPS), in which DPS was incorporated into beta-CD cavities, was prepared by a co-precipitation method. The tribological properties of the complex used as lubricant additive in PEG 600 aqueous solution were investigated by a four-ball tester. The complex exhibited better tribological properties than beta-CD under different loads, and also showed better anti-friction performance than DPS in the latter half of the test duration. The tribological action mechanism of the complex on a steel surface was studied according to the X-ray photoelectron spectroscopy (XPS) analyses. The beta-CD molecules of the complexes were decomposed into various molecular fragments and the DPS molecules were released under the friction condition. It revealed that thiolate and ferrous sulfide (FeS) films formed by DPS played a major role, and iron alkoxide and carbon deposition films formed by the friction fragments of beta-CD mainly exhibited anti-friction property on FeS-to-FeS interface. The interactions among different films led to the formation of a mixed boundary lubrication film. (C) 2013 Elsevier B.V. All rights reserved.