Tensile properties and deformation behavior of DP780 steel were studied using servo-hydraulic high-speed material testing machine, SEM and TEM. The effects of strain rate and the mechanism were investigated. The results showed that the strength and ductility of DP780 steel remained almost unchanged as the strain rate increased at strain rates lower than 10⁰ s^-1. When the strain rate was over 10¹ s^-1, the strength and the strain-hardening coefficient increased remarkably. Ductility of DP780 steel increased significantly at the strain rates ranging from 3~10¹ to 5~10² s^-1. The deformation resistance increased with increasing the strain rate due to the stronger short range resistance induced by the acceleration of dislocation motion in the ferrite matrix. Increasing strain rate up to 3~10¹ s^-1 resulted in a considerable increase of the amount of mobile dislocation, which was the main reason for the increasing uniform elongation and fracture elongation of DP780 steel at the strain rate ranging from 3~10¹ to 5~10² s^-1. Interface of ferrite-martensite in DP780 steel was the main location for pile-up of dislocation, crack initiation and propagation. The ability of inhomogeneous plastic deformation of DP780 steel increased due to the decreasing plastic strain energy difference between the ferrite matrix and ferrite-martensite interface and the consequent delaying initiation and propagation of microvoids at ferrite-martensite interface induced by the increasing work hardening degree of ferrite matrix with the increasing strain rate.