Abstract: Effects of the ferrite-martensite dual phase structures with different volume fractions and strength ratios on the strength and plasticity of the low carbon steel (0,11%C) and deformation behavior and fracture process of the dual phases were studied. Experiments indicate that the deformation in m artensite regions is less than the deformation of the bulk. The difference between the two relates not only to the strength of martensite, but also to the shape of the the martensite. Under low temperature tempering, the microcavities and crackings before fracture nucleate only in ferrite, the fractures are mixed in specimens undergoing tempering at higher temperature, however, the sites where microcavities and crackings nucleate have no preference between the two phases, the crackings often, propagate through martensite regions as well and the fractures are all dimple.