The paper investigates the fatigue damage processes of a welded joint made of 316L stainless steel plates by using the digital image correlation (DIC) technique. First, the strain partition around a surface defect evidenced very high value of strain up to 10% at the notch root during the tension stage, which explained the poor fatigue life of some welded joint specimen. In healthy specimen, a strain gradient from the base metal to the fusion zone in the range of 0.3–0.6% to 3% respectively was evidenced. Fatigue crack always initiated in the fusion zone where the deformation concentrated either early in the fatigue life at high stress or progressively by a mechanism of deformation transfer from the base metal to the fusion zone at low stress.