This study is focused on the experimental clarification of ductile cracking controlling mechanism in structural steel with heterogeneous microstructures. The structural steel with Ferrite-Pearlite two-phases with different strength in Vickers hardness was used. The ductile cracking tests were conducted under single tensile loading for round-bar specimens with/without circumferential notch in order to compare the ductile cracking behaviors under different triaxial stress state during loading. The observation of voids nucleation and growth behaviors up to ductile crack nucleation in the middle of the minimum cross-section of the specimens indicated that the damage for ductile cracking would be localization of plastic strain up to voids nucleation at the Ferrite-Pearlite boundary. This behavior was applied irrespective of the triaxial stress state during loading. Consequently, the main controlling factor for ductile cracking in the employed two-phase steel was found to be not growth of voids induced by larger inclusions, but nucleation of voids in softer phase (Ferrite phase) near Ferrite-Pearlite boudnary after large-scale plastic straining.