In this research, the failure behavior and toughness mechanism of ST44 sheet in the plane stress state have been investigated using the essential work of fracture method. In this research, ST44 steel sheet was used to make samples. To obtain the elastic-plastic mechanical properties of ST44 sheet, tail-shaped specimens were used for tensile testing. Then tensile samples with two deep edge grooves (DENT) were made to perform the essential work of fracture test. The samples were cut by laser in dimensions of      180 × 60 × 2 mm. In the middle part of each sample, two notches with a length of 9 to 19 mm were created, and the distance between the tips of these two notches was the length of the ligament. After performing the tensile test on the samples, the area under the force-displacement graph was obtained for each sample and with the help of regression, a line was drawn from the obtained points. The width obtained from the origin of the equation of the line is the specific essential work of fracture and is equal to 554.92 kJ/m². In order to calculate the critical opening of the crack tip and the critical opening angle of the crack tip, the maximum displacement diagram obtained from the force-displacement plot was drawn. From the drawn plot, the critical opening of the crack tip for ST44 sheet is equal to δc=2.5285 mm and the critical opening angle of the crack tip is equal to Ψc=0.33 rad.