Abstract: The elasto-plastic stress-strain analysis at notch roots of notched hardwares is discussed in this paper, proceeding from the modified Neuber's rule. The definition of the fatigue notch coefficient Kf is reconsidered and Kf is redefined by the modified Neuber's rule directly. It is pointed out that with the increase of the local plastic deformation, the Kf does not monotonously decrease, but, first decreases gradually and then increases gradually. The Kf would be larger than Kf（theoretical stress concentration coefficient） under greater plastic deformation. A varied Kf-Neuber's rule is presented and discussed. Both the stress-strain experiment analyses and the fatigue life tests indicate that the method presented in this paper is a better crack formation life prediction method with good accuracy.