The tensile process of 800MPa grade dual-phase steel was observed in-situ by SEM. The initiation and propagation of crack in dual-phase steel, the initiation of micro-pores, the deformation behavior of ferrite and martensite were analyzed in this paper. The results show that the main crack goes across the soft ferrite and around the hard martensite during its propagation in the beginning. When the applied stress is small, the micro-pores are easy to form inside the ferrite grain or on the interface of ferrite and martensite. The micro-pores can also form in the fracture site of martensite with the increase of applied stress. The fracture morphology of the dual-phase steel is dimple and the formation of fracture belongs to plastic fracture.