In the previous report the authors reported various conditions for the loss of ductility of the toe of the first pass side welding during finishing pass side welding due to thermal strain. But the present paper deals with the investigation carried out in order to clarify the relation between such phenomenon and fracture mechanism seen from the stand point of material mechanics. In other words, the authors sought the effects upon ductility of prestrain, aging, etc. by means of simple testing of round bar, etc. Moreover, concerning the notch at the toe of the first pass side welding, its relation with welding conditions and effects on fracture were studied. Further-more, root cracking conditions of one side multi-layer butt welding were also investigated. The results of these investigations were as follows : 1) The effects of tensile strain and compressive strain upon ductility differ. The former reduces ductility by raising flow stress from the stand point of material quality while the latter causes it by complicated causes such as changes of notch shapes, triaxiality, etc. 2) Notch shape changes according to welding position and electrode diameter and, the effects of such notch conditions upon crack initiation have been recognized. 3) Similar to the toe of fillet welding, the multi-layer welding root of the butt joint initiates crack in accordance with the strain due to subsequent pass welding and notch conditions.