The crack propagation behavior in fretting fatigue is divided into two stages, namely : the SI - and SII - stages. In the SI - stage, the crack propagation rate da/dN is very high, and decreases to a minimum with crack growth. In the SII - stage, the da/dN increases monotonicly with crack growth. The interpretation of this kind of crack propagation behavior has been rather difficult with a conventional stress intensity factor. In this paper the stress intensity factors under fretting fatigue stress conditions were calculated by BEM. Then the crack proagation behavior was analyzed by these stress intensity factors. The equivalent stress intensity factor for fretting fatigue Kelf = √(KIf2+KIIf2), where KIf is the stress intensity factor for mode I and KIIf is for mode II, was used to represent the da/dN in the SII - stage. The results agreed well with ordinany, or unfretting fatigue results. The revised equivalent stress intensity factor Kelf*=Kelf·√((a+a*)/a), where a is crack length and a* is the crack length at the end of the SI - stage, was available to represent the da/dN from the SI - to the SII - stage. These results could be widely applied to data for carbon steel and aluminum alloy.