The purpose of this study is to clarify the atomization mechanism of a spray injected into a low-pressure-field through a pintle-type electronic control injector, especially the change in spray characteristics dependent on the surrounding back pressure. In the experiments, fuel oil, that is, Shell-LAWS, n-pentane, or n-hexane, is injected with relatively low pressure into quiescent gaseous atmosphere at atmospheric pressure. The spray is observed by taking photographs at an arbitrary time and for variation of the back-pressure by using a 35-mm camera and a CCD camera system. The results show that the back-pressure has a great influence on the spray pattern and the atomization characteristics and that, in particular, the saturated vapor pressure of the fuel oil is the most significant factor. Spray pattern characteristics such as the spray angle, and atomization characteristics, such as the Sauter mean diameter, are almost constant in the range of back-pressure from atmospheric pressure to the vapor pressure. As the back-pressure is decreased to the vapor pressure, vapor bubbles start to grow due to flash boiling. Further decreasing the back-pressure below the vapor pressure results in an increase in the spray angle and in a marked decrease in Sauter mean diameter, since rapid expansion of the vapor phase may then atomize the fuel jet. Consequently, in the case of back-pressure below the vapor pressure, spray characteristics can be explained in terms of the growth rate of vapor bubbles corresponding to the pressure difference between the back-pressure and the vapor pressure.