The influence of nanoparticle size on the tribological performances of the SiO₂/PAG (Polyalkylene glycol) nanofluids under different lubricating condition were investigated. The silica nanoparticles displayed strong size-oriented adaptability to the lubricating conditions. Under lower frequency, the smaller nano-SiO₂ could significantly improve the tribological performances, this is due to the formation of tribofilm with stronger intrinsic mechanical properties. However, the lubricating oil containing large nano-SiO₂ showed better friction reducing property under higher frequency, it because the flow features of nanofluids has become a dominating factor affecting the tribological performance. What's more, compared with the nano-additives with the uniform size, the mixed nano-additives displayed better anti-wear and friction-reducing properties.