Abstract: Friction damper is an effective way to reduce the response of structure and it has been widely used to alleviate the resonant stress of blades.The friction damper is often modeled as a hysteretic spring.The stiffness of spring is just the damper's tangential stiffness.So it is very important to know the stiffness value of a real damper.In this paper,a new method to evaluate the stiffness of damper is proposed,by which the tangential stiffness of damper can be identified.The constitution of stiffness is studied systematically based on the experimental data.It is found that the value of tangential stiffness depends on both normal load and excitation.There is a fixed relationship among these three factors.