For the purpose of clarifying assessment of large strain unstable brittle fracture, studies have been made of the effect of specimen geometry, such as notch depth and plate width on general yielding deformation behaviors and fracture criterion of notched steels. Tension test have been done by the use of surface type through width notched specimen of mild steels mainly at temperature range at which large strain fracture occurs. In the range of 2a/ t ≤ 0.25 (a : notch depth, t : plate thickness) the gross section yielding of surface type notch specimen occurs preceding to net section yielding, because of the increase in plastic constraint at notch section due to both side notch. For shallow notched specimen, three different stage possibilities in plastic deformation behaviors can be considered; (1) small scale yielding, (2) general yielding (I) at which gross section yielding prior to net section yielding occurs, (3) general yielding (II) at which whole section yielding occurs, that is, full yielding stage observed in through-thickness type notched wide plates cannot be found. The critical COD of specimens with surface type shallow notch becomes almost constant value regardless of plate width in large strain fracture range. A linear correlation can be observed between non-dimensional COD, δ/a, and overall strain, e _∞. Although the proportional factor of δ/a- e _∞ relation decreases with increasing notch depth and plate width, the value of the factor obtained by through-thickness type notched wide plates is given as a maximum value.