Abstract: Dual-phase steel DP780+Z and low alloy high strength steel HC420LA are commonly used in automobile industry, the research about laser overlap welding was carried out in this paper by using fiber laser welding system. The microstructure and microhardness near the weld were analyzed, and the influence of different welding speed on microhardness of weld were also studied. Results show that the microstructure of the joint mainly includes fusion zone, heat affected zone and base material. The microhardness of HC420LA steel shows the increasing trend from base material to weld at constant laser power. However the changing trend of DP780+Z is first increasing and then decreasing. With the increase of welding speed, the microhardness of HC420LA increases, and that of DP780+Z first increases and then decreases.