This paper presents a formability analysis of tailor-welded blanks (TWBs) made of cold rolled steel sheets with varying thicknesses. Steel sheets ranging between 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, and 1.0 mm in thickness were used to produce TWBs of different thickness combinations. The primary objective of this paper is to characterize the effects of thickness ratios on the forming limit diagram (FLD) for a particular type of TWB. The TWBs chosen for the investigation are designed with the weld line located in the center of the specimens perpendicular to the principal strain direction. Nd:YAG laser butt-welding was used to prepare different tailor-made blank specimens for uniaxial tensile tests and Swift tests. The experimental results of the uniaxial tensile test clearly revealed that there were no significant differences between the tensile strengths of TWBs and those of the base metals. After the Swift tests, the formability of TWBs was analyzed in terms of two measures: The forming limit diagram and minimum major strain. The experimental findings indicated that the higher the thickness ratio, the lower the level of the forming limit curve (FLC) and the lower the formability of the TWBs. The findings also show an inverse proportional relationship between thickness ratios and minimum major strains. TWBs with a thickness ratio of close to 1 were found to have a minimum major strain closer to those of base metals. The effects of different thickness ratios on TWBs were further analyzed with a finite element code in a computer-aided engineering package, PAM-STAMP, while the failure criteria of the TWBs in the finite element analysis were addressed by the FLCs which were obtained from the experiments. However, the weld of the TWB in the simulation was simply treated as a thickness step, whereas its heat affected zones were sometimes disregarded, so that the effects of the thickness ratio could be significantly disclosed without the presence of weld zones. The results of the simulation should certainly assist to clarify and explain the effects of different thickness ratios on TWBs.