Composite laminates have been extensively used in the aerospace industry, especially for the fabrication of high performance structures. In order to ensure high reliability of the structure the actual mechanical properties of the material must be accurately predicted. The determination of stiffness parameters for complex materials such as fiber-reinforced composites is much more complicated that for isotropic materials. It is well known that material properties determined from the standard tests of small specimens, which are manufactured with similar technology as the real large structure, may differ from the actual material properties in laminated composite structure. In order to determine more accurate material properties of the structure the specimens are cut directly from the large stiffened composite panel, which before was tested for post-buckling. A curved stiffened panel with one stringer is cut out from the original 5-stringer panel. These panels were tested for vibrations in order to measure eigenfrequencies and corresponding modes. Natural frequencies of the curved stiffened panels were measured using a shearography technique. Experiments were performed with free boundary conditions on all edges of the plate in order to exclude influence of boundary conditions to the results of identification. Elastic properties of curved stiffened panel are determined employing the experimentally measured eigenfrequencies with free-free boundary conditions. The identification functional represents differences between experimentally measured and numerically calculated frequencies, which are dependent on variables to be identified. The identification procedure is based on experimental design and response surface approximations.