Abstract: The Dual-Phase steels are high strength steels with the formability of conventional steels of much lower strength ,the properties having been associated with a two phase structure of ferrite and martensite. The failure mode of Dual-Phase steel sheet during the forming processes was by plastic instability and voids formation .Forming Limit Diagram (FLD) and the volume fraction of voids were determined on a Mn-Si Dual-Phase steel with different volume fractions of martensite. The effect of voids formation on the critical failure strain has been discussed with the aid of damage mechanics. The critical failure strain along Forming Limit Diagram (FLD) is described reasonably well by the equation derived namely.