The fundamental physical processes of the spray breakup mechanism from the hole-type nozzle are examined In early 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, investigation has determined that the strong turbulence in the nozzle hole due to cavitation phenomena contributes greatly to the disintegration of the liquid jet. To reveal the mutual relationships, experiments were performed under conditions with varying length-to-hole diameter ratios L/D, and different inlet shapes and different internal shapes of the 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. Next, the effects of the internal flow in a diesel nozzle on the atomization of a spray were analyzed experimentally and numerically. Flow visualization studies were made using a transparent acrylic model nozzle.