Small bore pipes of secondary system in power plant are socket welded for connection. During welding process, weld defect is formulated by incomplete penetration during welding. Due to these geometric discontinuity region, structural fatigue life of socket welded pipe can be affected. In this paper, to predict fatigue life of socket welded pipe considering defect, structural stress is applied to evaluate stress at geometric discontinuity region. Also, equivalent structural stress and master S-N curve, which are derived from fracture mechanics, are proposed as an evaluation method to predict fatigue life of socket welded pipe considering defect.