Abstract: A fuel supply system in an off-road vehicle including front and rear wheels having wide tires, respectively, an engine mounted on a rear portion of a vehicle frame, and an exhaust pipe extending from the engine and disposed on one side of the engine transversely of the vehicle frame. The fuel system comprises a fuel tank assembly disposed on the other side of the engine transversely of the vehicle frame, and an air cleaner case disposed rearwardly of the fuel tank assembly. The vehicle frame comprises a plurality of welded pipe members. The fuel tank assembly has an outer peripheral flange supported at plural locations thereon by at least two of the pipe members of the vehicle frame. The fuel tank assembly has a cap for detachably closing a fuel inlet hole, the cap including a resiliently biased cover swingably mounted on an upper surface for covering a key slot from above.