Abstract: Water-soluble SiO₂ nano-particles were prepared from sodium silicate by a CO₂ precipitation method using polyethylene glycol as surface modifier and sodium polyacrylate as dispersant. Morphology and structure of SiO₂ nano-particles were investigated by TEM, XRD and FT-IR. Tribological properties of two water-based lubricants composed of water-soluble SiO₂ nano-particles with or without DDPN were investigated by a four-ball machine to determine the synergy of the nano-particles and DDPN. Wear scars on the steel ball surfaces were studied by SEM and AES. Results showed that SiO₂ nano-particles with a size distribution from 2 nm to 5 nm had no obvious aggregation. The water-soluble SiO₂ nano-particles had synergy effects with DDPN on wear resisting and friction reducing. Thus, the tribological performance of the blend was higher than that of the each component. The mixture of 0.15% SiO₂ nano-particles and 5.0% DDPN had the highest tribological properties with the pB value of 902 N and DWS of the steel balls of 0.63 mm. The water-soluble SiO₂ nano-particles had good self-repairing performance during friction.