In order to ensure the welded structure of low strength steel from the unstable brittle fracture at room temperature, the unstable fracture beyond general yielhing should be prevented. In this paper, the new material criterion, named “Strain Criterion”, based on the ductility of the structural element with defects is proposed for this fracture. Experimental studies on the effects of various factors such as specimen size, crack length, loading mode and so on reveals the followings; 1) Brittle-ductile transition temperature T_s of the tensile specimen with a notch is little influenced by crack length, specimen size, residual stress and local embrittlement in the vicinity of the notch tip. 2) Fracture overall strain of the tensile specimen with a notch beyond general yielding can be estimated as follows : e_F =δ _c /2π c Where δ _c is the critical COD and c is a half crack length. 3) Low strained brittle fracture initiation temperature T_L at which the fracture overall strain of the tensile specimen with a notch is equal to yielding strain of material used is the temperature that the critical COD satisfies the following equation : δ _c =2π c σ _Y / E 4) The testing method measured elongation in small notched tensile specimen is proposed as an evaluation method of materials based on strain criterion.