Stiffened composite plate has been widely used in many braches of engineering area and the demand of optimizing the cost of manufacturing is also very high. One of many approaches to minimize the cost is to optimize the weight of the structure. In this paper, an improved version of the Differential Evolution (DE) algorithm is adopted to solve for suitable values of the fiber angle and the thickness of the stiffened composite plate to achieve the structure with minimum weight. For computing the constrained conditions of stress and strain in the optimization process, the finite element analysis using the CS-DSG3 element is used. To verify the accuracy and the effectiveness of the algorithm, the numerical solutions obtained from the proposed method are compared with those of other available approaches.