Abstract: Zinc dialkyl phosphate (ZDP) was synthesized and its anti-wear properties as an additive in PAO were performed by using a four-ball tribometer. The morphologies and element analysis of the worn steel surfaces were investgated using scanning electron microscopy and X-ray photoelectron spectroscopy. ZDP exhibited lower friction coefficient but larger wear scar diameters compared to ZDDP. Tribological properties can be improved by using composite additive of ZDP and amine due to the formation of elements P and N containing tribo-film. The phosphorus content of the tribo-film was higher in comparison with that without amine.