Abstract: The effect of face abrasive wear by debris in sealed fluid on sealing performance was studied for Ugrooved mechanical seals typically used in a nuclear primary coolant pump. The results show that the fluid film stiffness and leakage rate would increase at initial stage of face wear, and the leakage rate increases with the number of worn Ugrooves. When sealing dam is penetrated by debris, fluid film stiffness will drop abruptly and leakage will increase sharply. Often, there are some worn cracks onto seal faces at the same time in practice, therefore such a seal will break down with great quantity of leakage due to only a depth penetrated scratch or a flaw.