An investigation on sti!ened plates has been conducted for the "rst time to determine the elastic parameters as well as the cross-sectional dimensions of rectangular sti!eners from experimental modal data and "nite element predictions, using model-updating technique. The problem is formulated as a global minimization of the error function, de"ned by the di!erence in modal properties as predicted from the "nite element modelling to that obtained &&experimentally''. The parameter estimation problem is solved using an iterative-gradient-based minimization algorithm and can start from di!erent randomly selected set of initial parameters. The Levenberg}Marquardt algorithm has been implemented to minimize the error function. Proper bounds are realistically chosen for the selection of the initial values of the parameters. The position, physical properties and orientation of sti!eners create considerable variations in the modal properties as compared with the bare plates of similar construction. This makes each of the sti!ened plate identi"cation problem rather unique. The position of sti!eners as well as its orientation makes certain measurement points more suitable for comparing analytical and experimental mode shapes. The optimum measurement set of co-ordinates, which can be used for the purpose of identi"cation, is thus case speci"c. The geometrical properties of sti!ener cross-section also a!ect the modes. Inclusion of di!erent sets of modes in noisy environment and the di!erences in the estimated parameters are studied. A few simulated examples are presented to investigate the uniqueness and convergence of results. The methodology is found to be very accurate and robust.