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It has been pointed out that cavitation inside a fuel injector plays an important role in the fuel spray characteristics, which affects the thermal efficiency and exhaust gas emissions. Recently, it was pointed out that when needle lift is low, not only orifice wall cavitation but also single string cav-itation may occur in the mini-sac and valve-covered orifice (VCO) nozzles, which increases largely the spray cone angle. However, the mechanism and condition of the appearance of single string cav-itation have not been clarified yet. In this study we carry out high-speed visualization of orifice wall cavitation and single string cavitation in various transparent injectors with a single orifice and a simple rectangular sac, whose gap Z, sac width W, orifice diameter D, liquid kinematic viscosity ν, and mean liquid velocity V in the orifice are varied, to investigate the governing dimensionless parameters and to examine whether we can quantitatively predict the formation criteria of single string cavitation or not. Moreover, particle image velocimetry (PIV) analysis is conducted to measure the velocity distribution, to clarify the flow structure, and to quantify the circulation in the sac. As a result, we conclude that regardless of the shape and size of the fuel injectors, the length of cavitation in an orifice can be predicted by using the modified cavitation number σc. The appearance of single string cavitation and circulation in the sac are possible to be predicted quantitatively by using a dimensionless Re1 proposed in this study, which is the ratio among the inertial and viscous forces.