Fretting frequently challenges the integrity of structures exposed to fatigue loading. The mechanics of the fretting fatigue loading must be understood in order to analyse what are the important parameters in the crack growth process of fretting fatigue. Mode I stress intensity factors of cracks perpendicular to the surface have previously been used to describe the growth of fretting fatigue cracks. Numerous experiments show that fretting cracks initially do not grow perpendicular to the surface, and the loading situation suggests a multi-axial fatigue crack growth regime. The scope of this investigation was to quantify the error of assuming a crack path perpendicular to the surface, and to model the crack path using different mixed mode crack growth criteria. After the initial mixed mode stage I growth seen for all fatigue cracks, the fretting cracks were observed to grow along a path inclined beneath the fretting pad, but where AKH = 0. Hence, mode I crack growth behaviour seemed to dominate the development of the fretting cracks, while the influence of mode II loading was limited. In cases where a large contact pressure is applied and a high degree of inclination of the fretting crack is observed, the real crack path was to be used in order to take account of the crack closure effect. a =a*+a' a' C da /dN integration path in eq. ( 2) range of nominal stress smooth specimen fatigue limit crack deviation angle used in the growth direction criteria angle of crack inclination as defined in Fig. 1.