In this study, we perform dislocation-crystal plasticity simulation of Dual Phase steel (DP steel) and discuss the mechanism of microstructure-dependent strength and ductility. DP steel is widely used duplex stainless steel composed of ferrite and martensite. DP steel, which consists of hard ferrite and soft martensite, can cover a wide range of strength and ductility. Various combinations produce a wide variety of properties. A number of papers have reported the improvement of the mechanical strength owing to the reduction of grain size. Especially, ultra-fine grain metals show not only high strength but also high ductility [1]. For DP steel, however, the mechanical contributions of ferrite and martensite to the strength have not been clarified in terms of the grain size dependency. Therefore, their effects and behaviours have been unclear. To solve this problem, we conduct dislocation-crystal plasticity simulations [2] for DP steel. The dislocation-crystal plasticity simulation enables us to analyse the effect of grain boundaries between ferrite and martensite phases. The DP steel models with different grain sizes of matrix ferrite are prepared, while the grain sizes of martensite are the same between two models. They are applied to the simulations. Then, we divide the effect of the mechanical property of ferrite and martensite precisely by the use of the results obtained by dislocation-crystal plasticity simulation. The stress-strain curves with equivalent stress and a stress component in loading direction are analysed. The distributions of stress, strain and dislocation densities are calculated to consider the roles of ferrite and martensite. As the result, we find that the grain size dependency of ferrite, i.e., the deformation in the smaller grain size model becomes homogeneous, which leads to the high strength contribution by martensite. REFERENCES [1] Tsuji N., Kamikawa N., Ueji R., Takata N., Koyama H., Terada D., Managing Both Strength and Ductility in Ultrafine Grained Steels, ISIJ international, Vol. 48 (8), pp. 997-1010, 2008. [2] Aoyagi Y., Shizawa K., Multiscale crystal plasticity modeling based on geometrically necessary crystal defects and simulation on fine-graining for polycrystal, International journal of plasticity, Vol. 23 (6), pp. 1022-1040, 2007.