Abstract: To evaluate the friction force of secondary seal ring seal in the reactor coolant pump (RCP) mechanical seal, a test setup was designed and the friction force of the secondary seal ring was measured at the different reciprocating frequencies and amplitudes. The influences of frequencies and amplitudes on the friction force were analyzed. The results show that the friction force increases followed by a decrease with the increase of reciprocating amplitudes. The reciprocating frequency shows little effect on the friction force. The alternating shear stress on the metal surface contacting with the secondary seal ring leads to the fretting due to the fatigue.