Premature failure at the shear pin of stainless hot rolling mill caused by increasing productivity and fluctuation of roll torque raises significant downtime cost. To reduce downtime cost, UNSM (Ultrasonic Nanocrystal Surface Modification) treatment is applied to the neck groove of shear pin as the alternative to existing solutions. This treatment prevents premature failure, increases service life and reduces the number of replacements. Proper process variables of UNSM technology which give the strongest fatigue strength are determined by comparing with S-N curve of rotary bending test result. Surface roughness, hardness and compressive residual stress on three kinds of test specimens are analyzed together with SEM analysis of fracture surface. Real shear pins are produced by the property of highest fatigue strength and installed to the hot mill stand for validation. They come to pass as twice as the original operation cycle.