Tribological properties of magnesium silicate hydroxide superfine powder additive and the compound additives of magnesium silicate hydroxide superfine powder and MoDTC in PAO4 base oil were studied on the MMW-1A type vertical universal friction and wear tester.The friction surfaces were observed,characterized and analyzed by means of metallographic microscope,SEM and EDS.The results show that the magnesium silicate hydroxide superfine powder can improve the friction and antiwear properties of PAO4 base oil,but there are still obvious scratches and pits on the frictional surface.The compound additives composed of 025% magnesium silicate hydroxide superfine powder and 03% MoDTC can obviously improve the tribological properties of PAO4 base oil,copmared with PAO4 base oil and the base oil with single additive of magnesium silicate hydroxide superfine powder,the friction coefficient is reduced by 3275% and 1787%,respectively,while the wear scar diameter is reduced by 5316% and 3202%,respectively.The compound additives have significant effect on improving the worn surface morphology,which can form a repairing layer composed of C,O,Mo,S,Mg and Si elements on the friction surfaces.The two kinds of additives complement each other during the friction process and play a synergistic role in promoting each other.