Stiffened plates are common components in many structures, from plate girders and box girder bridges to ships and offshore structures. Stiffened and unstiffened plates buckle into doubly curved surfaces which complicates the analytical solution. Finite element methods allow direct analysis of stiffened plates without requiring the solving of complicated differential equations. However, such analysis are usually time demanding processes especially regarding 3D-modelling and mesh creation in finite element programs for complex geometries. The ultimate load bearing capacity of plates and stiffened panels will depend on whether the considered plate or stiffened panel is an integrated part of a ship side, bulkhead structure etc. or whether they are a part of a girder web with free membrane boundary conditions. Integrated thin plates in ship decks, bottom or ship sides etc. can carry load beyond the ideal elastic buckling load, while plates with membrane stress support has limited overcritical strength. The buckling code DNV-RP-C201 has been worked out by DNV (Det Norske Veritas) and describes two different, but equally acceptable methods, for buckling and ultimate strength assessment of plated structures considering the above-mentioned reserve load bearing capacities for integrated panels. The first method, as given in ``Part 1'', is a conventional buckling code for stiffened and unstiffened panels of steel, and ``Part 2'' is a computerised semi-analytical model called PULS (Panel Ultimate Limit State) which can be used as an alternative to Part 1. Calculation of the girders according to Part 1 may be carried out by assuming whether or not the stiffened plate is effective against transverse compression stresses. For slender stiffened plates, for instance, the load carrying resistance in the direction transverse to the stiffener may be neglected. The present study presents the results of an investigation into buckling of stiffened steel panels by means of Finite Element Analysis (FEA) as well as the above-mentioned methods recommended by DNV. The related general theoretical backgrounds are presented briefly in the first chapters. In order to further explore the differences between the methods of buckling analysis recommended by DNV and nonlinear finite element buckling analysis, a capacity assessment of a stiffened panel is carried out separately based on these methods and the corresponding results are compared with each other. By utilising DNV recommendations for buckling assessment of stiffened panels the author comes also with a proposal of weight optimisation for a stiffened steel panel. For performing analysis in accordance with Part 1, the program STIPLA is used. Both STIPLA and PULS are recognised by DNV. For performing nonlinear finite element analysis, the program ABAQUS is used and the analysis procedure is presented in detail. In the current work buckling strength and ultimate capacity of a stiffened steel panel are predicted by performing analysis in the above-mentioned programs and the corresponding capacity curves are plotted. The differences between different methods of analysis are discussed and also the eventual disadvantage, if any.