An investigation was performed into the effect of mechanical stress relieving on low load fracture of welded wide plate, reported by Prof. H. Kihara and his research group, and the analysis of the effect was made, based on the concept of the flow and fracture stress curves. In the analysis, the history of plastic stress and strain at the tip of the notch was pursued by taking into account of so-called Bauschinger's effect and also introducing the concept of Orowan's plastic constraint for a sharp notch. And a reasonably assumed value was used for the strain concentration factor of the notch. The result of the investigation showed that this new method of analysis was succeeded in clarifying quantitatively the mechanism of recovery in fracturing load due to mechanical stress relieving.