Recently, the weight reduction of vehicle influences its environment problems and performances. It is a trend that a lot of parts have been currently changed to an aluminum alloy from steel materials. In this study, the shape optimization using an orthogonal array is performed to determine the design of the knuckle which is a part of suspension system. With the material of the weight reduction was achieved by satisfying the constraints of a strength requirement. The orthogonal array of $L_{18}$ is introduced to find the optimum design variables that considers the shape of the knuckle. The characteristic function composed of the objective and the construct is defined to the feasibility. Comparing to the weight of the initial design with steel materials that of optimum design with aluminum alloy material is reduced by 60%.