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The aim of this paper is to develop a micromechanical model to quantitatively upscale the damage initiation of dual-phase steels under various stress states from micro to macro and reveal the underlying mechanisms of the damage initiation dependency on stress states from a microstructural level. Finite element (FE) model based on the real microstructure of a DP600 steel sheet is employed by representative volume element (RVE) method. Several numerical aspects are also discussed, such as mesh size and discretisation feature of the phase boundary. The plastic strain localisation theory is applied to the RVE modelling without any other damage models or imperfections. Three typical stress states, uniaxial tension, plane-strain tension and equibiaxial tension, are considered to investigate the influence of the stress state on damage initiation. The quantitative evaluation of the damage initiation for three stress states obtained from the RVE simulation shows the dependency on both stress triaxiality and Lode angle. The results are further compared to the experimentally calibrated damage initiation locus (DIL) and a fairly good agreement is achieved. From this study, the general physical understanding of the effect of stress states on damage initiation is explored and the method for quantitative analysis of the damage initiation in a microstructural level is also established. The microstructure heterogeneity is considered as the key factor that contributes to the damage initiation behaviour of the dual-phase steel.