A dialkyldithiophosphate modified MoS2 nanoparticle LB film was prepared on copper substrate, The tribological behaviors of the film against steel were evaluated using a one-way reciprocating friction tester. The structural changes of the film during the rubbing process were investigated with a Fourier transform infrared microscope and by X-ray photoelectron spectroscopy. It was found that the nanoparticle LB films decreased the friction coefficient and increased the wear resistance of the copper substrate significantly. Analyses indicate that formation of a transferred film on the steel counterface, and the occurrence of the tribochemical changes during friction process, mainly involved disordering and partial decomposition of the surface-modified agent. The nucleus of the MoS2 nanoparticle plays a role in load carrying and antiwear, especially after partial decomposition of the surface-modified agent with increased friction times. A shearing effect was mainly responsible for exciting the tribochemical change.