Laser butter welding is implemented by 6 kW fiber laser on 800 MPa dual phase cold rolled steel sheet with 1.5 mm thickness. The microstructure evolution of laser welding joint, its effect on microhardness and fatigue properties are studied. The results show that the weld joint consists of weld zone (WZ), coarse-grain heat affected zone (CGHAZ), fine-grain HAZ (FGHAZ), mixed-grain HAZ (MGHAZ) and tempering zone (TZ). The microstructure of WZ and CGHAZ are martensite, and columnar crystals morphology is retained on original austenite grains in WZ, whereas polygonal original austenite grains are observed in CGHAZ. The microstructure of FGHAZ and MGHAZ are ferrite and martensite, and finer microstructure is obtained in FGHAZ. The microstructure of TZ consists of ferrite and tempering martensite. The microhardnesses of both MGHAZ and TZ are lower than that of base metal, and they form the softening zone of the welded joint together. Tensile fracture is in the base metal because of the narrow soften zone (0.4 mm) and low microhardness drop (~6.8%). In tension-tension fatigue tests (stress ratio is 0.1), the fatigue limit of base metal and weld joints are 545 MPa and 475 MPa respectively. Fatigue fractures are not in softening zone. The fatigue crack propagates along the phase interface between ferrite and martensite in base metal. Whereas, the fatigue crack of welded joint occurs on the austenite grain boundaries or martrnsite lath in WZ, and propagates along the intersection of columnar original austenite grain boundaries in the center of WZ, which cuts off martensite lath beam to propagate.