To provide lifetime predictions of metal pieces under fretting-fatigue loading, the crack initiation process has to be understood. However, there is no consensus in the literature over the crack nucleation length. The objective of this study is to understand the link between fretting crack nucleation and microstructure to resolve this question of the critical crack nucleation size. For this purpose, cylinder/plane fretting tests were performed to plot the evolution of the crack length as a function of tangential load amplitude. Finally, a link between the critical distance, evaluated by inverse method, and the grain size is established for different crack nucleation lengths. The investigation suggests that the critical distance evolution depends on the considered defect size.