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