This paper presents the analysis of atomization and vapourization processes in a flash boiling spray based on the experimental results, for the subject of injection systems in the suction manifold of gasoline engine. Two kinds of liquid fuel, n-pentane and n-hexane, were injected into quiescent atmosphere at room-temperature and low-pressure through a pintle type injector with electronic control. The spray characteristics of both fuels below various atmospheric pressures were investigated in detail by taking photography. Then, in the region of flash boiling, where the back pressure was below the saturated vapour pressure of fuel, the bubble nucleation process due to the flash boiling was modelled by both the measurement results of bubble and the nucleation rate equation using the degree of superheat of the liquid fuel. The results show that the bubble nuclei initiation occurs inside the orifice in the injector owing to the marked pressure reduction near the orifice edge. Further, the fuel vapourization process was assessed by the bubble growth calculation in vapour cavitation phenomena, the fuel evaporation due to boiling from the liquid film surface and the evaporation process from the film surface owing to heat transfer between the ambient gas and the liquid film. Accordingly, the transient change in the bubble diameter and the vapour mass fraction inside the spray are estimated quantitatively for each back pressure. The calculated results, such as the shape of fuel film jet and the fuel droplet diameter in the spray, agree well with the experimental results.