Flashover occurring under repetitive pulsed voltage perhaps may not happen at the first pulse of applied repetitive pulses. Thus, studies of flashover stressing time (FST) are important to comprehend the flashover mechanism under nanosecond-pulsed voltage. In our experiments, the pulsed power source was SPG200, based on semiconductor opening switch (SOS). Solid dielectrics employed were polymethyl methacrylate (PMMA) and polyamide 1010, and liquid dielectric was transformer oil. The dispersion of FST was statistically large at invariable or variable voltage amplitude and frequency of repetitive pulses. Furthermore, the dispersion of FST decreased, and concurrently, the odds of that flashover arising at the first pulse of repetitive pulses increased, with the increase in the voltage amplitude of repetitive pulses whose frequency was invariably retained. The flashover tended to occur at the tail of the pulse waveform, and the mean value of FST was found to gradually reduce. Moreover, the extent of reduction was prominently decreased after about 200 Hz, when the pulse frequency increased, which simultaneously restricted the variation of the voltage amplitude of the applied repetitive pulses. Thus, it was demonstrated that longer FST is the ultimate factor, which induces lower flashover field strength under repetitive nanosecond pulses. [ABSTRACT FROM AUTHOR]