Significance of investigation of ductile crack initiation characteristics in the evaluation of structure of integrity is on the increase. It is significant to predict ductile crack initiation in plastic forming of metal materials. A lot of examinations about ductile crack initiation behavior under high stress triaxiality for structural steels have been conducted using notched round bar tensile specimens. Satisfactory examination under low stress triaxiality, however, has not necessary been conducted thus far. Ductile crack initiation characteristics of aluminum alloy A 5083-O and high strength steel HT 950 under low stress triaxiality have been investigated using plate tensile specimens in this study. The critical equivalent plastic strain as a function of the stress triaxiality at ductile crack initiation has been estimated using FE analyses. The ductile crack initiation is triggered by secondary void nucleation under low stress triaxiality, and it is controlled by initial void growth under high stress triaxiality for A 5083-O. The ductile crack initiation is triggered by void nucleation under the wide range of stress triaxiality. The critical diameter of void at ductile crack initiation varies with stress triaxiality for both materials. The critical equivalent plastic strain at ductile crack initiation under low stress triaxiality is seemed to be overestimated by extrapolation using curve fitted to test results under high stress triaxiality on the assumption that critical void diameter would be constant.