Abstract: The extreme pressure and antiwear(EP AW) properties of cis and trans dihydroxy fatty acids were studied by Optimol SRV tester. Results show that hydroxy fatty acids had poor EP property, but possessed some AW ability. In addition, the AW funciton of transdihydroxy fatty acid was obviously better than that of cis dihydroxy fatty acid. The surface of the wear scar was analyzed by Fourier Transform Infrared Reflection Absorption Spectroscopy (FT IR RAS). It was found that cis dihydroxy group could easily adsorb on the rubbing surface. This led to shortening the linear alkyl chain which could adsorb on the surface. On the basis of the experimental results, the different adsorption antiwear models for cis and trans dihydroxy fatty acids were proposed. The models could well explain the experimental results and help in developing novel hydroxy fatty acid based AW additives.