Carmakers have tried to lower the vehicle weight for raising fuel efficiency. This trend involves a trade-off with the vehicle stiffness. In automobile interior parts, the thickness has needed to be decreased for the weight reduction but this makes the stiffness worse.A new approach for improving the stiffness due to the weight reduction is required and various optimization methods at early development stage have been introduced currently. However, it is difficult to apply optimization for the interior parts since many interior parts' structures generally depend on the design. But as studying the structure in detail, we discovered some factors that affect the performance without depending on design.The door trim is selected for optimization item because it has many characteristics of automobile interior parts. In our case study, the factors that improve the performance of door trim without changing design are considered as fastener position and flange rib layout. The optimization process for door trim was established. Size optimization is used for Fastener position and Topology optimization is used for Flange rib layout.As a result, the 1st mode frequency is improved by about 5% and thermal displacement is reduced by about 25% in comparison with the initial model.