In the optimization of the complicated structure, the computational cost increases rapidly as the number of design variables increases. And we need a great amount of calculation and time on problems of modified dynamic characteristics of large and complicated structures. In this paper, the multi-level optimization is used, which decreases computational time and cost. The multi-level optimization methods used in this study are as follows : In the first step, the dynamic characteristics is controlled for the simplified two-dimensional model of stiffened plate by sensitivity analysis and quasi-least squares methods. In the second step, the cross-section of the stiffener is decided so that the weight is minimized under needed constraints. In the third step, the three-dimensional model is made based on the results of the first step and the second step, and then confirmation calculation of the dynamic characteristics is carried out. The vibration characteristics of the stiffened plate is examined, and some main vibration modes are studied. The simplified equation for the main vibration mode of stiffened plate is proposed. In the multi-level optimization, the dynamic characteristics of the stiffened plate is changed by the proposed simplified equation and the effectiveness of the method is examined. It is shown that the results are effective in the optimum modification for dynamic characteristics of the stiffened plate.