Abstract: In this paper, influences of microstructure and substructure on plastic deformation and fracture of low-carbon martensite steel have been studied. There are ε-FexC and Fe3C in quench-autotempered martensite. There is orientation difference between adjacent marten site laths. Microcracks were nucleated at the interface between martensite laths and interface between martensite packets. Fracture units are martensite lath and martensite packet.