The tribological behaviors of hydroquinone bis(diphenyl phosphate) (HDP) and tricresyl phosphate (TCP) for polyalkylene glycol (PAG) were evaluated at 200 degrees C. Results showed that HDP could effectively reduce the friction coefficient and prevent wear of sliding pairs during the test. The best tribological properties of HDP achieves at the concenteration of 4 wt%. At this level, the wear volume of the lower disk can be reduced by a factor of 2.4 and the load-carrying capacity can increase from 500 N to 930 N, with respect to pure PAG. Furthermore, HDP exhibited better antiwear (AW) and extreme pressure (EP) properties than TCP, despite the fact that their friction-reducing performances were similar. Boundary lubrication films composed of FeOOH, Fe2O3, FePO4, and polyether compounds were formed on the worn surface, which resulted in excellent friction reduction and AW performance.