Abstract: In situ tensile tests in a scanning electron microscope(SEM) were carried out using single-edge notched specimens for two types of steels:a pure high-strength steel(HSS) composed mainly of ferrite and a small amount of pearlite,and a wheel steel composed mainly of pearlite and a small amount of proeutectoid ferrite. Regardless of the specimen thickness meeting the plane strain condition or not,the process of deformation and cracking for the pure HSS starts at ferrite slip,then continuously propagates and finally results in ductile fracture; the small amount of pearlite has nearly no impact on the overall deformation process; and the fracture surface is composed of dimples. However,for the wheel steel,when the specimen thickness is thin,which means that it does not meet the plane strain condition,plastic deformation occurs firstly in proeutectoid ferrite,even though its amount is very small,and then microcracks initiate and grow along the interface of proeutectoid ferrite and the pearlite colony,and finally evolves into discontinuous microcracks. The fracture surface fractograph presents both dimple and quasi-cleavage characteristics. For a wheel steel specimen with the thickness meeting plane strain condition,plastic deformation and cracking mainly occurs in the pearlite colony,which results in cleavage fracture.