In the present experimental study, the flash atomization mechanism has been studied in detail, focusing mostly on the two-phase effluent flashing mode. Both the internal flow and external flow patterns (that is, the flow patterns before and after the discharge) were considered simultaneously. Photographs of sprays from transparent nozzles and the circular single-hole nozzles were examined. Depending on the injection conditions, a bubbly, slug, or annular flow pattern appears inside the nozzle before the discharge, and it was confirmed that the internal flow pattern governs the external flow (spray) behavior outside the nozzle. With a longer nozzle hole or with a higher degree of superheat, the spray drops are smaller and more uniform because of the active bubble formation inside the nozzle. In support of the present interpretation of flash atomization characteristics, the spray angle and drop sizes were measured and compared with previous results.