In this paper, a new approximate method based on analytical formulas is proposed to estimate the ultimate strength of stiffened panels by investigating collapse mechanisms of the stiffened panels. In this respect, firstly, a series of detailed elastoplastic large deflection Finite Element Analysis (FEA) is performed to investigate the collapse behavior and the ultimate strength of the stiffened panels. In the analysis, the initial deflections are considered in the form of thin-horse mode plus overall buckling mode for the plates, and flexural buckling mode plus tripping mode for the stiffeners. On the basis of the observed FEA results, an efficient method is proposed to estimate the ultimate strength of stiffened panels based on Elastic Large Deflection Analysis (ELDA) with the initial yielding concept. In the ELDA, the deflection modes are defined as the sum of overall buckling mode plus local plate buckling mode. The welding residual stresses are not taken into account in this study. The ultimate strength is predicted by examining yielding at several critical points. The calculated results by the proposed method and the nonlinear FEA are compared, and a very good agreement is obtained for all collapse scenarios investigated.