Today, need for using light-weight components and decreasing cost of manufacturing processes is inevitable for various sections of industries. So in sheet metal forming processes new concepts of fabricating consolidated sheets or in other words Tailor Welded Blanks emerged. Friction Stir Welding is one the methods for manufacturing TWBs and has numerous advantages over fusion welding methods for joining aluminum sheets. In present study, TWBs made by friction stir welding of 6061-T6 and 5754-O aluminum alloys were studied. The effect of different tool rotational speeds and welding speeds on mechanical properties and microstructural characteristics of dissimilar joints were evaluated. The results showed that in an appropriate range of speed combinations, an optimum rotational speed exist at which maximum strength is achieved. Regarding welding speed, greater strength is attained at higher speeds. The microstructural analyses confirm that above the optimum rotational speed, the grain size of the nugget zone increases which results in decreasing tensile strength. With regard to welding speed, increasing speed will result in grain size reduction and consequently higher tensile strength. Positioning Al 6061 on advancing side of the dissimilar joints lead to improved mechanical properties compared with positioning on retreating side. It can be due to improvement in mixing of the base materials as a wider nugget zone obtained at such situation. It is notable that the degree of such improvement in ductility is much more remarkable than strength, which is valuable regarding formability concerns. In the next section, the formability of post-rolled dissimilar sheets was assessed by bulge test. It was shown that the TWBs that are rolled perpendicular to the weld line, the formability of Al 5754 side relatively reduced comparing with Al 6061 side. It is in contradiction to the fact that Al 5754 base sheet has much more ductility comparing with Al 6061 base sheet.It seems that the Al 5754 is much more affected by the cold rolling process. The TWBs that are rolled parallel to the weld line, all of the rapture locations, occur in the weld of Al 6061 side that are similar to the fracture locations in tensile tests. Also it is shown that the formability decreases at higher rotational speeds and improves with increasing welding speed. At rotational speed of 1600 rpm and welding speed of 63 mm/min, comparing the results of bulge test with tensile tests shows that although the ductility is highest in tensile test but the formability reduced in bulge test compared to lower rotational speeds.Also the joint produced at speed combination of 1000 rpm and160 mm/min, not only had greatest tensile strength in tensile test, but also exhibited highest amount of formability in bulge test. So it is concluded that both tensile strength and ductility are important and should be simultaneously considered concerning formability of TWBs. Keywords: Tailor welded blank, Dissimilar friction stir welding, Aluminum, Formability, Bulge test