In most buses, there are more than 2 steps for the passenger to enter. But buses with a low floor have no steps for easy entry. To make the floor a low height, the CNG tanks were shifted to the roof in the low floor buses. Because of the heavy mass on the roof, the front body and the side structure must be reinforced to sustain the increase in mass. The mass of the low floor bus is much higher which means the fuel economy is lower than the normal buses. Therefore weight reduction is essential in low floor buses to increase the fuel economy. The target component for weight reduction is the center frame. To reduce the weight of the center frame, the topology optimization technique is applied.