The flow and cavitation phenomena inside the nozzle sac play an important role in the fuel atomization process, especially at the end of the injection, which may cause high-temperature gas ingestion consequently leading to coke formation inside the orifice. It is of great difficulty to reveal the flow and cavitation morphology inside fuel nozzles during the fuel injection process since the nozzle sac is enclosed by a thick steel wall and the whole process takes place in a few microseconds. Therefore, a dedicated synchrotron radiation X-ray phase contrast imaging technology was developed by optimizing sample-to-detector distances and image processing methods, and the image intensity and contrast were enhanced to visualize the transient cavitation morphology. The flow and cavitation morphology at three different injection pressures were recorded and compared. The results show that the cavitation bubbles and gas ingestion process inside the nozzle sac can be clearly observed revealing a two-stage flow dynamics process occurring inside the sac. At the first stage, the cavitation bubbles appear in the sac around the needle tip surface due to the sudden pressure decrease; at the second stage, the ambient gas was ingested through the orifice due to the increase of the vacuum degree in the sac, leading to a rapid burst of the cavitation bubbles. The ingested gas replacing the cavitation bubbles then occupies the most space of the sac at the end of the injection.