In this paper, the effect of the size of a weld defect “lack of penetration” remaining in the welded butt joint of mild steel on brittle fracture initiation characteristics was investigated by using wide tensile test specimens. Three types of part-through weld defect notch are tested. Two of them are the surface notch and the internal notch with limited extension to the both directions of thickness and breadth. The other one is the internal notch through breadth on weld line. From the results of surface and internal notch tests, a change in crack extension mode from the direction of breadth into that of thickness with an increase in notch depth was observed clearly. A complicated phenomenon that the brittle fracture initiated at low stress levels in the through-breadth internal notch test specimen, in which the brittle cracks propagated only in the direction of thickness was observed. But it is remained to analyse theoretically the results of these part-through notch tests, based on the energy condition theory for brittle crack initiation, and further based on the close examination for the state of actual stress on the notch section. However, as it is considered that the deep notch test is practically effective method to investigate the brittle fracture initiation characteristics for the welded joint under one of the severest conditions, additional investigations which can be applied to the through-breadth internal notch specimens will provide another valuable basic data with respect to the brittle fracture initiation characteristics at low stress levels for the welded joint.