A fuel tank structure of straddle type vehicle in which a wide space below the fuel tank can be secured and a clearance portion for avoiding the fuel tank interfering with the vehicle body side can be downsized. The fuel tank is supported on and disposed above a main frame with a fuel pump being installed to a bottom plate of the rear portion of the fuel tank. A maintenance space formed below the fuel pump when the rear portion of the fuel tank is lifted in a state wherein an installation portion of a front bracket installed to the front lower portion of the fuel tank is rotatably supported on the main frame. The fuel tank is configured such that the installation portion of the lower end of the front bracket is disposed below the bottom plate, simultaneously, and is installed to the main frame with a bolt.