A fourball machine was used to evaluate the antiwear and extreme pressure properties of tricresyl phosphate (TCP), Zinc dialkydithiophosphate (ZDDP) and their synergistic effect. The results indicate that TCP possesses better antiwear performance than ZDDP, but ZDDP possesses better extreme pressure performance than TCP. TCP exhibits good antiwear synergism with ZDDP under the low load. ZDDP can improve the extreme pressure property of TCP.