Tailor welded blanks (TWBs) offer several notable benefits including decreased part weight, reduced manufacturing costs, increased environmental friendliness, and improved dimensional consistency. To take full advantage of these benefits, however, one needs to overcome the reduced formability of TWBs and be able to accurately predict the failure modes depending on the thickness ratio between the thicker and thinner sheets of a tailor welded blank with similar base materials early in the design process. In this paper, we studied the effect of the thickness ratio on the limit dome height of TWBs by using finite element method and experiments, analyzed the effect of TR on the failure mode of TWBs. A concept of the critical thickness ratio was presented to predict the failure modes of TWBs. The results show that the rapture originates on the weld of TWBs when the thickness ratio is less than the critical thickness ratio and on the thinner side when the thickness ratio exceeds the critical thickness ratio.