The tribological properties of different addition and particle size silicon dioxide powder as additives in lithium base greases were investigated on four ball friction tester,the influence of surface modification on tribological properties of silicon dioxide powder was analyzed.The morphologies and the elements of the worn surface were analyzed by means of optical microscope and EDX（Energy Dispersive X ray Detector）,and the anti wear mechanism was discussed.The results show that the anti wear and friction reducing properties and the extreme pressure properties of the lithium base greases with silicon dioxide powder are improved in certain tribological conditions.The surface modification can improve the dispersion property of silicon dioxide powder in lithium base greases,but it has less influence on the tribological properties.Results of the EDX analysis show that there are characteristic elements on the surface of the ball,which indicates the silicon dioxide powder can react with the frictional surface to form the lubricating film with good tribological properties.