Abstract: CdS nanoparticles were prepared in a room temperature ionic liquid, 1-methyl-3-hexyl-imidazolium tetrafluoroborate. X-ray powder diffraction and transmission electron microscopy were used to characterize the structure of the CdS nanoparticles. The results indicate that the average diameter of the nanoparticles is about 15 nm. The nanoparticles have hexagonal structure. The tribological behaviors of 1-methyl-3-hexyl-imidazolium tetrafluoroborate ionic liquid and its complex system which containing CdS nanoparticles were studied on a four-ball machine. The results indicate that the ionic liquid containing CdS nanoparticles shows better tribological behavior than pure ionic liquid. CdS nanoparticles increased the decomposition of anion existing in ionic liquid, and their rolling/sliding reduced the friction.