The mechanical properties of the DP780 specimens from two different product lines are compared first. It is shown that the mechanical property differences of the same grade DP steels are evident. Then the finite element model of the uniaxial tensile test is established. The principle strain acceleration evolution of the failure point is used to predict the necking and fracture of the DP780 during uniaxial tensile accurately with the combination of tensile test and FEA. It is found that the forming limit of the same grade DP steels is significantly different. Finally the effects of strain hardening exponentn and strength coefficientK varying on the forming limit are analyzed quantitatively. It is shown that the range of the strain hardening exponentn has an obvious influence on the forming limit while the influence of the strength coefficientK is not distinct. The elongation and limit strain at necking and fracture can be improved by increasing the strain hardening exponentn.