Dialkyldithiophosphate-modified copper nanoparticles (DDP-nano Cu) were prepared by means of a redox surface modification technique. The structure of DDP-nano Cu was investigated with an ultraviolet-visible light spectrometer and a transmission electron microscope. The elemental composition of DDP-nano Cu was determined with an elemental analyzer. The dispersing behavior of DDP-nano Cu in various organic solvents was compared with nonmodified copper nanoparticles. The antiwear ability of DDP-nano Cu as additive in liquid paraffin was examined with a four-ball machine. It was found that DDP-nano Cu could well disperse in some organic solvents and had good antioxidation stability in air. The modified copper nanoparticles used as additives in liquid paraffin showed different antiwear ability depending on their particle size. The difference in the antiwear behaviors of nonmodified and DDP-modified copper nanoparticles is explained by their different interactions with the surface of the friction pairs during the friction process.