To make premium brand and image, we are considering about added convertible project continuously. Therefore put this study to prepare convertible project. In order to secure torsion stiffness of a convertible vehicle which is without roof, using the analyzed principle of coupe and find parts which are affects at the torsion stiffness. To improve this portion in priority, designed convertible vehicle. The competition vehicle weight increased 113.6kg, but this study convertible weight increased only 55.3kg compared to base vehicle(base coupe). Also achieve the torsion stiffness goal. This study is identify the characteristics of convertible vehicle and suggested the convertible body structure which is carry over with base vehicle.