Abstract: Cucurbit7uril (CB7) was synthesized and characterized by nuclear magnetic resonance, elemental analysis and Fourier transform infrared spectroscopy. Its thermal property was determined by simultaneous thermogravimetry. The tribological capacity of cucurbit7uril as lubricant additive in water-based fluids (H₂O and H₂O/ethylene glycol) for steel/steel contact at room temperature was evaluated by an Optimol SRV-IV tester. X-ray photoelectron spectrometer (XPS) analysis was carried out to explore the lubrication mechanism. The results suggest that cucurbit7uril has high thermal stability with an onset decomposition temperature of more than 360 ℃. Cucurbit7uril as lubricant additive can effectively improve the friction-reducing and anti-wear properties of water-based fluids. When cucurbit7uril was added, the friction coefficient decreased from about 0.30 to 0.20 and the wear rate reduced by more than 50%. Based on the XPS analysis, it can be deduced that the good lubrication of cucurbit7uril was mainly attributed to the generation of physical adsorption films on the sliding surface through electrostatic attraction between the carbonyl groups of cucurbit7uril and the positive charge occupied steel surfaces.