Abstract: Biodiesels were synthesized by the transesterification reaction of cottonseed oil and methyl alcohol and ethyl alcohol, respectively, in order to study the lubricating properties of biodiesel. The biodiesel/diesel mixtures were then obtained by mixing the prepared biodiesels and commercial diesel. The lubrication properties of the obtained mixtures were studied on a four-ball tribological tester. The results showed that the synthesized biodiesels could improve the lubrication properties of diesel obviously. The increase of the biodiesel had a positive effect on the lubrication properties of the mixtures. Moreover, the lubrication properties could be affected remarkably by the different alcohols. The extreme pressure (PB value) of the mixture was increased by 94.1% and 29.4% by adding 20.0 vol% methyl ester biodiesel and ethyl ester biodiesel, respectively. The biodiesel from ethyl alcohol could improve the lubrication properties more than that from methyl alcohol. Free fatty acids can also affect the lubricity of biodiesel. The study lays the foundation of application of biodiesel.