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.