In recent automobiles, tailor welded blanks using high strength steel sheets with various combinations of thicknesses and laser welded butt joints are being widely used to reduce weight of vehicle bodies without any decrease of their stiffness and crash performance. It is well known that underfilled weld beads are formed in such butt joints due to large gaps between steel sheets in laser welding process and the underfilled weld beads deteriorate mechanical properties and metal formability. However, the influence of the underfilled weld beads on fatigue properties of the weld butt joints is not sufficiently revealed, even though the fatigue properties are practically important for actual service of automobiles. In addition, the fatigue properties of butt joints with a step cross-section are necessary to build the design standard for tailor welded blanks. Therefore, in this paper, tensile tests and fatigue tests have been conducted for the laser welded butt joints of different thickness steel sheets with various underfilled weld beads in order to clarify their fatigue properties. Also FEM analyses have been performed to study the stress distribution, especially the stress concentration, around the weld metal in the butt joints. It is observed that the butt joints with underfilled weld beads and a step cross-section significantly deteriorate the fatigue strength, although their effects on tensile strength are small.