The purpose of this investigation is to clarify atomization mechanism of a high-speed liquid jet issuing from a single-hole nozzle. In previous research, it was believed that atomization of the liquid jet was caused by the interfacial forces existing between the issuing jet and the surrounding gas. However, it has been determined that the strong turbulence in the nozzle hole due to cavitation phenomena contributes greatly to the disintegration of the liquid jet. In order to reveal the mutual relationships, experiments were performed under conditions ranging from decompression to high ambient pressures by using acrylic nozzles with various length-to-hole diameter ratios L/D and different inlet shapes of the nozzle hole, close to the hole diameter of an actual nozzle. As a consequence of this study, it has been determined that the primary factor in atomization of the liquid jet is the disturbance of the liquid flow resulting from cavitation phenomena.