This study presents a new design approach which can optimize position and size of a rockfall protection wall with the help of a numerical simulation. In the proposed framework, sufficient numbers of rockfall paths with information of kinematic energy are accumulated by rockfall simulation. A protection wall is then virtually overlapped on the simulated results, and values of a safety function are checked under the different combinations of design parameters. A response surface of the safety function is then approximately constructed, and a cost function is defined to express the penalty in unreasonable situation. Once the two functions are obtained, an optimization problem can be formulated. In order to demonstrate the capability of the proposed optimal design approach, a numerical example is presented in this study. The obtained result indicates that the proposed framework has the possibility of developing optimal design of rockfall protection walls.