The tribological properties of Mo S 2 microsized spheres ( MS - Mo S 2 ) with diameter of 0.5 – 3 μ m modified by self-prepared surfactant quaternary ammonium salt of 2-undecyl-1-dithioureido-ethyl-imidazoline (SUDEI) as an additive in base oil 500 SN were investigated and compared with those of commercial colloidal Mo S 2 ( CC - Mo S 2 ) on a four-ball tester and an Optimol SRV oscillating friction and wear tester in a ball-on-disk contact configuration. The worn surfaces of the bottom flat disk were examined with scanning electron microscopy and X-ray photoelectron spectroscopy. It was found that the Mo S 2 microsized spheres product was a much better extreme-pressure additive and antiwear and friction-reduction additive in 500 SN than commercial colloidal Mo S 2 ( CC - Mo S 2 ) . Under the appropriate concentration of 0.1% and 0.25% for MS - Mo S 2 and CC - Mo S 2 and the load of 400 N , the friction coefficient of MS - Mo S 2 /oil and CC - Mo S 2 /oil decreased about 25.0% and 12.5% and the wear volume loss decreased about 50.4% and 12.9% compared with the pure base stock. The boundary lubrication mechanism could be deduced as the effective chemical adsorption film formed by the long chain alkyl ( C 11 H 23 ) and active elements (S and N) in the surfactant SUDEI and tribochemical reaction film composed of the tribochemical reaction products.