In situ tensile test has been performed with the electron back-scattering diffraction (EBSD) technique for characterizing the deformation heterogeneity at microscopic level of a duplex stainless steel consisting of austenite and ferrite. It was observed that, as deformation proceeded, the fraction of low-angle boundaries continuously increased and strain gradient developed at some grain boundaries and twin boundaries, as well as in the interior of some grains. The in situ experiments quantitatively captured the change of grain-orientation-dependent plastic behavior in respective phases and the strain partition between duplex phases as a function of applied strain. Using a visco-plastic self-consistent (VPSC) model incorporating the accommodation of micromechanical properties of grains with different orientations in two phases, the evolution of microstresses/microstrains at various length scales was simulated and discussed in detailed within the material undergoing plastic deformation. The experimental observations are well explained by the VPSC model. The present investigations provide the in-depth understanding of anisotropic micromechanical behaviors of the duplex steel.