One of the most important aspects of Finite Element based soft wares is the ability of prediction of the location of damage propagation and crack initiation under different condition of loading. One of the most common damage models used in ABAQUS software is ductile damage model. In the present research, the most important damage models are studied and the method of their application in FEM soft wares is explained. Then the ductile damage model has been investigated and the input parameters for this model assisted simulation have been determined. The triaxiality stress is the most effective parameter on the fracture strain in ductile damage model and the damage parameter is calculated regarding to the values of triaxiality stress and equivalent strain during loading time. The damage parameter has a magnitude between zero and one. If the magnitude equal to zero; it means that the element is undamaged or virgin and the rapture of elements is shown by one. Therefore the magnitude of damage parameter can be predicting damage propagation and crack initiation. The material which is used in this study is St12 steel. Some tension tests on notch samples with different notch radius have been done in order to specify the material’s damage constants. The effect of the triaxiality stress on the fracture strain has been studied for different notch radiuses through the tests and the equation which specifies the relation between the fracture strain and triaxiality stress is obtained and calibrated for St12 Steel. Moreover, the ductile damage model has been applied for related simulations in order to clarification of validity of the results of previous experiments. Therefore in the first step, the specimens applied for calibration of the equation of fracture strain, simulated based on the ductile damage. The results of the simulation have a good compatibility with experimental observation about the prediction of the time and location of fracture. In the second step, a butterfly specimen and a transient specimen have been designed and manufactured as the tension test sample. Then the tension test has been simulated for mentioned specimens. In the case of butterfly specimen, triaxiality stress on the fracture section approximately equal to zero. In this situation the shear stress has the main role in fracture phenomena. In the case of transient specimen, triaxiality stress on the fracture section approximately equal to 0.5 and the fracture phenomena is occurred by reason of the shear and tension stress combination. In the final the damage propagation has been studied in an industrial five stand cold rolling mill. The prediction of damage propagation in this process is in good agreement with experimental observation. Keywords: Ductile damage, Damage parameter, Triaxiality stress, Fracture strain