The flash boiling by fuel heating is a useful method to control the time spatial spray characteristics such as atomization of droplets, vaporization and air-fuel mixture density. It is one of the important phenomena for a direct injection gasoline engine (D.I.S.I) as a next generation powertrain. This report focuses on flash boiling phenomena under superheated fuel spray condition using fuel heating to understand its physical phenomena with scattered light method, schlieren photography and Super High Spatial Resolution Photography (SHSRP) which is our original analysis method. Fuel was iso-octane and a single hole nozzle were used for the purpose of basic phenomenon analysis. The influence on spray shape can be shown by schlieren photography. Spray droplet diameter and spray dispersion at the nozzle exit are observed by super high spatial resolution photography which is our original development technique. As a result, spray dispersion and atomization were controlled and improved drastically by flash boiling. It was clarified that fuel spray shape greatly varied by heating more than saturated temperature and not only atomization of the whole spray is fine, but also a spray core region decreases. In the heating condition that is slightly lower than critical temperature, a shock wave “MACH DISK” was confirmed at nozzle exit.