The effect of shot peening on torsional fatigue strength of induction hardened shaft with hole notches was investigated. The fatigue strength of shot peened specimens is improved in the case of deep case depth especially. In the case of shallow depth, the cracks initiate at the hardened / non-hardened boundary. On the other hand, the fracture of deeply hardened specimen originates at the surface layer. The compressive stress is given to the surface layer of hole by shot peening. The improvement of fatigue strength is caused by the compressive residual stress. Fatigue crack propagation in the residual stress field was analyzed by the fracture mechanics approach, and the fatigue strength of shot peened specimens was calculated. The calculated data is in agreement with experimental data.
