Introduction of compressive residual stresses on the fatigue-loaded components is one of the techniques followed to improve the fatigue life of industrial components. Oil jet peening is a surface modification process for the introduction of compressive residual stresses. A high-pressure oil jet is made to impinge on the surface to be peened. Preliminary studies were carried out on the medium carbon steel at the oil pressure of . The compressive residual stress induced on the surface of unpeened and oil jet-peened AISI 1040 steel was and , respectively. Fully reversed cantilever bending fatigue behaviors of medium carbon steel in both under peened and unpeened conditions were evaluated at room temperature. Oil jet-peened specimens exhibited superior fatigue performance compared to the unpeened specimens. Fractographical analyses were carried out for specimens broken at several tested stress levels using optical microscope.