The isooctane spray characteristics were experimentally investigated under flash boiling condition which represents the part load operating condition for modern gasoline engine. Various tests were carried out with back pressure ranging from 0.2 bar to 1 bar and ambient temperature varying between 20 degrees C and 100 degrees C. A long distance microscope together with an ultrahigh speed camera was employed to capture the spray development in the near field to study the primary breakup characteristics. The study was performed by using a diesel common rail injection system so that the influence of hydraulic force can be investigated. It was found that flash boiling led to dramatic radial propagation due to the explosion and collapse of the vapor bubbles, significantly boosting the atomization. The strength of vapor bubble explosion in the near field tended to be strongly affected by the flow regime in the nozzle. Besides, higher injection pressure led to larger cone angle during the initial injection stage but smaller cone angle during the quasi-steady stage due to the dominance of the hydraulic force. (C) 2016 Elsevier Ltd. All rights reserved.