Abstract: In a motor vehicle having an engine mounted in an engine room, an exhaust tube system extending from the engine toward a rear end of the vehicle and a fuel tank positioned near the rear end of the vehicle, there is proposed an improved under body structure of the vehicle, by which the fuel tank is effectively cooled during running of the vehicle. The under body structure comprises an under cover positioned below the engine room. The under cover is formed with a longitudinally extending groove whose opening faces downward to contact a relatively low temperature air. A floor panel is formed with a longitudinally extending tunnel whose opening faces downward. The tunnel receives therein the exhaust tube system without contacting the same. A heat insulating plate extends in and along the tunnel in a manner to define in the tunnel an air flow passage which is insulated from the exhaust tube system. The air flow passage has a rear open end directed toward the fuel tank.