An underbody structure of a vehicle includes a floor panel of which rear portion is raised to form a rear floor, a pair of left and right rear side frames extending generally along a longitudinal axis of the vehicle at the rear of the floor panel with a specific distance held between the left and right rear side frames across the vehicle, and a rear cross member extending widthwise across the vehicle and interconnecting forward end portions of the left and right rear side frames. A fuel tank is mounted between the left and right rear side frames, and the rear cross member is located under the floor panel at the front of the fuel tank.