Abstract: Monodisperse Ag nanoparticles were prepared in quaternary W/O microemulsions containing Span 80-Tween 80, liquid paraffin and 1-butanol. It was found that mass ration of Span 80 and Tween 80 had an obvious effect on the morphology and size distribution of the Ag nanoparticles. Morphologies, structure and thermal properties of the silver nanoparticles were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), thermogravimetry (TG) and differential thermal analysis (DTA). The results show that the Ag nanoparticles have cube crystal structure; exhibit uniform size distribution and have an average size of 6 nm at the mass ratio of Span 80∶Tween 80=7∶3; and are coated with long chain fatty acid through carboxyl. H2O and 1-butanol in microemulsions were removed through vacuum distillation, and Ag nanoparticles were dispersed stably in liquid paraffin. The tribological properties of liquid paraffin containing Ag nanoparticles were evaluated on a four-ball test machine. Results showed that the Ag nanoparticles in liquid paraffin exhibited od antiwear property and could improve load carrying capacity of base oil remarkably.