The spray structure and vaporization processes of flash-boiling sprays in a constant volume chamber under a wide range of superheated conditions were experimentally investigated by a high speed imaging technique. The Engine Combustion Network’s Spray G injector was used. Four fuels including gasoline, ethanol, and gasoline-ethanol blends E30 and E50 were investigated. Spray penetration length and spray width were correlated to the degree of the superheated degree, which is the ratio of the ambient pressure to saturated vapor pressure (pa/ps). It is found that parameter pa/ps is critical in describing the spray transformation under flash-boiling conditions. Three distinct stages namely the slight flash-boiling, the transition flash-boiling, and the flare flash-boiling are identified to describe the transformation of spray structures. As the superheated degree increases, for the slight flash-boiling stage, the penetration length linearly and slightly increases, the near-field spray width keeps constant or slightly increases, and the far-field spray width linearly and slightly decreases; for the transition flash-boiling stage, the penetration length slightly decreases, the near-field spray width slightly increases, and the far-field spray width significantly decreases to the minimum; for the flare flash-boiling stage, the penetration length, the near-field spray width, as well as the far-field spray width significantly increase. The addition of ethanol to gasoline could decrease the penetration length, and increase the spray width. The penetration length and spray width for E50 is smaller and larger respectively than those for E30 under the same superheated degree, especially on the flare flash-boiling stage.