The analysis of fracture surface which shows a sequence of occurrences due to crack propagation allowed make associations between the fracture s relief and applied stress. The fractured surface may indicate things like active load lines and the way of fracture process. The objective of this work is to compare the resistance to crack propagation of SAE 1020 and SAE 4140 steels, thus enabling an evaluation of their performances in mechanical and / or structural components when defects are present. The canvases were taken with a scanning electron microscope and it possesses qualitative character. The samples were submitted to the fracture test with different geometric proposals in order to promote comparison for different modes of crack propagation The analysis of the surface of the fracture, which shows a succession of occurrences due to the propagation of the crack, allowed to associate the relief of the fracture with the applied tensions. The events expressed by the fractured surface can indicate the lines of acting loads, the process of the fracture, Among others. The first comparison was made with the SAE 4140 steel with test pieces with and without lateral notch, where the CP with precrack presented deformation, whereas the CP with side groove showed a crack more straight and this is due to the state restriction of flat tension caused by the lateral notch. The second comparison occurred between precracked specimens of the SAE 1020 and 4140 steel, where the specimen of the 1020 steel deformed considerably, making the fracture process gradual due to its ductile characteristics, while the fracture of the CP 4140, if developed suddenly and generated little deformation. By comparative analysis, the 1020 steel showed higher toughness at 4140.