Nowadays, tailor welded blanks (TWBs) have a great role to play in saving of cost, vehicle weight, and fuel consumption in automobile industries. A TWB constitutes blanks with different thicknesses and/or coatings and/or mechanical properties welded together prior to the stamping process. Owing to low formability of TWBs, determining and predicting the forming limit diagram (FLD) of these blanks enables manufacturers to produce non-imperfection parts. In this research, a modified type of Marciniak-Kuczynski (M-K) model was applied to determine the FLD of a TWB consisting of interstitial free (IF) galvanized steel sheet with different thicknesses (0.8 and 1.2 mm). Also, the effect of three yield criteria, namely Hill48, Hosford79, and BBC2002, on the predicted FLDs was investigated. During the analyses, the materials were assumed to follow the Hollomon work hardening rule. The experimental FLDs were determined utilizing the Hecker method. It was observed that the proposed method can properly predict the experimental FLD of a TWB with longitudinal welds and thickness ratios (TRs) greater than one, if proper parameters are being selected. Furthermore, the best prediction took place when the BBC2002 yield criterion was applied.