To investigate microstructure failure in ferrite-martensite dual phase steel,in-situ observations were performed on multiple plate DP800 specimens during uniaxial tensile tests. Microstructure evolution of the observed region was investigated in details. The experimental data showed that micro-cracks in various regions differed in the initiation time,and micro-failures mainly occurred from the locations with typical characteristics of stress concentration( i. e. ferrite interiors,the interfaces of ferrite-martensite grains and the martensite-martensite interfaces). Growth of micro-crack generally experienced the following stages: cracking from martensite boundaries,tiny particles in ferrite interiors,or martensite interiors,propagating in ferrite,bypassing martensite boundaries,or passing through martensite-martensite interfaces,finally ending on martensite boundaries. Martensite was one important source of micro-failure and changed the propagation of micro-cracks significantly. Microstructure deformation was inhomogeneous in the stage of plastic deformation. To investigate microstructure failure in ferrite-martensite dual phase steel, in-situ observations were performed on multiple plate DP800 specimens during uniaxial tensile tests. Microstructure evolution of the observed region was investigated in details. The experimental data showed that micro-cracks in various regions differed in the initiation time, and micro-failures mainly experienced from the locations with typical characteristics of stress concentration (ie ferrite interiors, the interfaces of ferrite-martensite grains and the martensite-martensite interfaces). stages: cracking from martensite boundaries, tiny particles in ferrite interiors, or martensite interiors, or martensite interiors, propagating in ferrite, bypassing martensite boundaries, or passing through martensite-martensite interfaces, finally ending on martensite boundaries. Martensite was one important source of micro-failure and changed the propagation of micro-cracks significantly. Micros tructure deformation was inhomogeneous in the stage of plastic deformation.