Abstract: Lanthanum diakyladithiophospate (LaDDP) and boric acid ester (OB) were synthesized. The tribological behaviors of the synthetic products as additives in ISOVG68 machinery oil were investigated on a four-ball machine, while the synergistic tribochemical effects of the two additives were explored based on the analyses of the worn steel surfaces by means of X-ray photoelectron spectroscopy and Auger electron spectroscopy. It was found that both the LaDDP and OB as additives functioned to significantly improving the friction-reducing,antiwear, and extreme-pressure properties of the base stock, which was attributed to the formation of a boundary lubricating and protective film composed of iron oxides and B_2O_3 or of iron oxides, FeS, FeSO_4, and FePO_4, by way of the tribochemical reactions among the additives,the atmospheric O_2, and the frictional pair. The combined additive of the LaDDP and OB was more effective to increasing the friction-reducing, antiwear, and extreme-pressure properties of the base stock, than the individual additive. This was because the tribochemical reaction products of the LaDDP acted to catalyze the decomposition of boric acid ester and promote the diffusion of the resulting B-containing products into the sub-worn-surface. It was therefore concluded that the LaDDP and OB had synergistic friction-reducing, antiwear, and extreme-pressure properties actions, which determined the better tribological behaviors of the combined additive than each individual one.