As cavitation inside diesel nozzles can improve the spray characteristics, it has long been a hot issue. And together with the increasing attention of biodiesel, it is essential to identify the difference of cavitating flow characteristics between diesel and biodiesel. What’s more, the hydraulic flip phenomenon and cavitating flow with decreasing injection pressure hasn’t been studied. Based on this, cavitating flow inside transparent nozzles of diesel and biodiesel fuels with increasing and decreasing injection pressure was investigated in this paper. Experimental results showed that are quite different from the disappearance of it and it is harder to disappear. Biodiesel and longer nozzle orifices were hard for the hydraulic flip phenomenon to occur, and the disappearance of hydraulic flip phenomenon has great influence on the spray cone angle and the discharge coefficient.