This study focused on scale effects on the in-nozzle flow regimes based on different scaled-up diesel injector nozzles, and the influences of the various flow regimes on spray characteristics. Optical results showed that flow regimes inside three different sizes of diesel nozzles went through single-phase flow, cavitation inception, cavitation shedding, wake cavitation, air suction, super cavitation, and hydraulic flip flow. Then the flow regime transition characteristics in three nozzles were analyzed. Cavitation inception number (K) in these nozzles were not the same because of the scale effect. Meanwhile, the average cavitation shedding frequency decreased with the increasing of the scaling factor of the nozzle, but the modified Strouhal number (St) was almost the same. Moreover, the decrease of the cavitation number (K) caused the reduction of the average frequencies of cavitation shedding and air suction. In addition, the length of cavitation region showed a strong inverse correlation with the length of air suction region. The collapse of air suction, which was mixed with the Cavitation or was pushed out the orifice rapidly, significantly influenced the spray cone angle. The Characteristics of various flow regimes in different scaled-up diesel injector nozzles were analyzed in this study, which provided preliminary exploration to the in-nozzle flow characteristics from the scaled-up transparent diesel nozzles to real diesel nozzles.