The tribological properties of P-containing antiwear (AW) and extreme pressure (EP) additives in the sliding of steel against aluminum alloy were investigated by Optimol SRV tester. It was found that phosphates reduces the friction coefficient of base stock. However, all P-containing AW and EP additives produce larger wear volume than base stock. The relationship between tribological properties of the additives and their chemical reactivity with aluminum was also found. The acting mechanism of P-containing additives in the lubricated aluminum-on-steel system was proposed.