Fretting fatigue is correlated with plain fatigue in order to develop a method to estimate fretting fatigue life from plain fatigue data. Fretting fatigue experiments as well as plain ones were conducted to obtain fatigue life data at various conditions. Finite element analyses were conducted to evaluate the Smith-Watson-Topper (SWT) fatigue damage parameter around crack initiation location. It is revealed that the SWT in fretting fatigue decays exponentially away from the surface. Moreover, a correlation function exists that relates the gradient of normalized SWT at the surface to the maximum SWT ratio of plain fatigue to fretting fatigue at the same life. It is demonstrated that equivalent SWT for fretting fatigue, which is determined from the correlation function, can be compared directly with plain fatigue data for estimation of fretting fatigue life.