The crude peptide was purified from Acaudina molpadioides, a widely distributed marine species, and further membrane separation afforded three peptides with different molecular weight. All three peptides show excellent solubility in water-glycol base fluid and high thermal stability. The tribological experiment shows that peptides are efficient in friction-reducing and worn surface protection. Especially, the peptide with low molecular weight can remarkably improve the extreme pressure capacity of base fluid. XPS analysis indicate that the tribofilm is composed of multiple iron oxides, nitrogen-containing organic compounds and so on. The abundant polar groups in peptide are also related to the formation of physical tribofilm on the surfaces. The results demonstrate peptides have the potential to become a novel eco-friendly aqueous lubricant additive.