Ferrite-martensite dual phase (DP) steels are widely used in the automotive industry. However, the mechanical response of DP steels is complex due to the huge difference in micro-deformation and hardening behavior between ferrite and martensite. In this study, the microstructure deformation and hardening behavior of DP600 steel were predicted by using conventional mechanism-bases strain gradient (CMSG) plasticity model. Firstly, the metallograph of DP600 steel during in-situ uniaxial tension test were recorded, and the strain distribution of each phase was calculated by using digital image correlation (DIC) analysis software. Subsequently, the observation area was analyzed with the representative volume element (RVE) method based on the CMSG model in various stress states, such as uniaxial tension and biaxial stretching. The experimental and simulation results show that (i) the deformation of the DP steel mainly depends on ferrite, and the micro-strain distribution calculated by CMSG model is consistent with the experimental analysis by DIC method, (ii) the strain of martensite is much less than the ferrite, which makes the high strain gradient produced at the phase boundary of ferrite-martensite, and increasing the region's geometrically necessary dislocations (GNDs) rapidly, (iii) GNDs have important effect on the hardening of ferrite at the phase boundary. In addition, the strain concentration of ferrite in martensite gap will decrease the ductility of DP steel.