Polyethylene glycol derived carbon quantum dots nanofluids were synthesized via a slow thermal oxidation process. The size of carbon quantum dots was ca. 2 nm and had a decreasing trend with the increase of oxidation time. When used as lubricant in a diamond-like carbon film/bearing steel interface, the nanofluids achieved an ultra-low friction coefficient (μ ≈ 0.02), much lower than that of original polyethylene glycol (μ = 0.12). The worn surface analyses revealed that the nanofluids could effectively inhibit the tribo-oxidation of steel counterpart that occurred under original polyethylene glycol lubrication, and hence reduced the abrasion component of friction. Especially, the poly-hydroxyl carbon dots and oxidized polyethylene glycol species in nanofluids induced a hydroxyl-rich sliding interface via their tribochemical reactions with friction surfaces, which promoted the adsorption of polyethylene glycol molecules on sliding surfaces. Along with the mild corrosion wear of steel counterface, this shifted the boundary lubrication to a mixed/film lubrication regime, thereby achieving an ultra-low friction coefficient. The above results suggest that the polyethylene glycol derived carbon quantum dots nanofluids should be a quite excellent candidate lubricant for solid–liquid synergy lubrication based on diamond-like carbon films.