The super high strength, ductility and one-dimensional system of multi-walled carbon nanotubes (MWCNTs) make them become ideal additives for liquid lubricants. Although the MWCNTs have been added in kinds of lubricants, the influences of the length and diameter of the MWCNTs have not been studied systemically when they are used as lubricant additives. In this study, a series of MWCNTs with different diameter and length are modified by oxygen functional group and long-chain alkyl, and applied as additives of water and liquid paraffin (LP) respectively to investigate how the length and diameter of the MWCNTs influence their tribological properties. The results indicate that the short length MWCNTs samples showed better tribological properties than the long length MWCNTs samples, and the middle-diameter (10-20 nm) MWCNTs sample presented the best antiwear performance either in water or in LP. The established mechanisms are that the appropriate diameter (10-20 nm) and length (0.5-2 mu m) make the MWCNTs-5 easier to enter the clearance of the friction pairs, play the role of rolling bearing between friction pairs, and fill in the cracks to mend surface damage of the friction pairs.