Abstract: Two kinds of organic borate esters containing thiadiazole group,referred to as BPCD and BPTT,were synthesized and their structures were characterized by infrared spectroscopy,element analysis and mass spectrometry. The thermal stabilities were good. Tribological behaviors of the two additives in lithium-based grease (noted as LBG) were evaluated by a four-ball friction and wear tester. The results show that the two synthesized compounds can significantly improve the extreme-pressure,anti-wear and friction-reducing properties of LBG. The action mechanism was studied by soft X-ray absorption near-edge structure and X-ray photoelectron spectroscopy. The results of XANES and XPS analyses indicate that a protective tribofilm,which mainly consisted of B₂O₃,FeS,FeS₂,FeSO₄,iron oxide and N-containing organic compounds,contributed to the excellent tribological properties.