Mesoporous carbon nanoparticles (MCNPs), with a particle size of 15–40 nm and pore size of 3–5 nm, were conveniently synthesized in ultrahigh yield (91.7 ± 1.5 wt%) by one-step carbonization of polyoxyethylene sorbitan trioleate (Tween 85) in H₂SO₄/H₃PO₄ mixed solution at a mild temperature (150 °C). The MCNPs showed particularly high dispersion stability in polyethylene glycol (PEG200) because their surfaces were abundant in long alkyl chains and ester groups. Considering that PEG200 is a kind of commonly used synthetic oil, the tribological properties of the MCNPs as a lubricant additive for PEG200 were evaluated in steel/steel contact and ball-on-plate reciprocating modes. The MCNPs exhibited the best friction-reducing and antiwear abilities when their concentration was 0.7 wt%, that is, adding 0.7 wt% MCNPs caused the friction coefficient and wear volume of PEG200 to reduce by 49.2% and 71.3%, 49.1% and 67.2%, 48.0% and 48.0%, 47.9% and 23.6%, and 44.4% and 8.1% at loads of 50, 100, 150, 200 and 250 N, respectively. The above results demonstrated that the antiwear function of MCNPs obviously reduced with increasing load despite their friction-reducing function only decaying slightly. Moreover, the performance of MCNPs was barely attenuated when the friction duration prolonged from 20 to 200 min, suggesting their long service life. Wear surface analysis implied that the MCNPs as an additive not only formed physical absorption films on the rubbing surfaces, but also showed rolling, polishing and mending effects under an appropriate load, directly accounting for their outstanding lubrication functions.