Cavitating flow in diesel injector nozzles affects subsequent atomization behavior and then spray characteristics which is decisive for diesel engine performance and pollutant formation.The visualization experiment research on the real-size optical transparent nozzle under high fuel injection pressure is still in its infancy.In this paper,particular attention was focused on the flow characteristics in the real-size diesel nozzle sac and orifice which were the subject to some conjecture.A comparative experimental study has been conducted to analyzing the evolution of cavitation inside diesel nozzle.When an injection signal send out from the ECU,the needle lift up,flash lamp flashing and an image of the nozzle flow was recorded under the injection pressure of 30,40 and 50MPa respectively,and the experimental results showed that the conditions of flow inside the nozzle were similar.The higher injection pressure,the earlier the cavitation inception occurred.The bubble “ sucktion ” from orifice exit at the end of injection and the bubble “ discharge ” at the next injection initial stage were observed.During period of closing of the needle under the higher injection pressure,the cavitation in the nozzle were presented “ banding ” and became to the “string cavitation” eventually.For the injection pressure of 60MPa,some images obtained during the needle motion.The larger spray angle(θ >28°)occurred at about h =0.36~0.48 during the period of opening or closing of needle symmetrically and the string cavitation from needle induced by the vortex flow appeared at some moments of those periods,and also the spray angles were increased remarkably when the string cavitation occurred.