Surface modified (NH4)3PMo12O40nanoparticles were synthesized in a mixture solution of waterethanol with cetyl trimethyl ammonium bromide as a modification agent. The structure of nanoparticles was characterized by TEM, XRD, FTIR, DSC, TGA, etc., and their tribological properties as additives in liquid paraffin were investigated with a four-ball tribometer. In the preparation process, Na2HPMo12O40·14H2O was previously synthesized according to a literature. Firstly, ammonium chloride and cetyl trimethyl ammonium bromide were respectively put into the mixture solvent of distilled water and ethyl alcohol in a flask with stirring, then, the solution of Na2HPMo12O40·14H2O was dropped into the flask, immediately, a yellow precipitate was produced. After 1 h of continuous stirring and 12 h of standing, the yellow precipitate was collected by filtering and was rinsed with alcohol and water, individually, desiccated at room temperature. The yellow powder (marked as sample a) was the expect surface modified (NH4)3PMo12O40nanoparticles. Non-modified (NH4)3PMo12O40particle (sample b) was prepared with the same procedures as above, except that cetyl trimethyl ammonium bromide was not used. The dispersity of sample a and sample b were evaluated in solvents such as chloroform、benzene、liquid paraffin, and distilled water, at room temperature. The results show that sample a disperses in organic solvents, but it doesn′t disperse in water, and sample b doesn′t disperse in organic solvents or water. It indicates there are organic groups on the surface of sample a. TEM graphic shows that the mean graininess of sample a is about 20 nm, and the average grain-size of sample b is over 1μm. This suggests that the modification agent can restrict the congregation of (NH4)3PMo12O40nano-cores. The FTIR absorption spectra of sample a indicate that (NH4)3PMo12O40nanoparticles have a Keggin structure, and confirm the existence of organic groups. The XRD pattern confirms there are (NH4)3PMo12O40nano-cores in sample a. The DSC and TGA curves of sample a show the decomposition temperature of (NH4)3PMo12O40nano-core is about 300℃. The tribological tests show that the synthetic nanoparticles as additive in liquid paraffin improve the antiwear and friction-reduction ability of the base oil, and the most appropriate additive concentration is 0.25 %. According to our inference, this is attributed to the generation of a surface protective film, by way of tribochemical reaction.