Abstract: Studying optical characteristics in the course of flashover in transformer oil is an effective way to explore breakdown mechanism of transformer oil. Optical signals were irregular pulses of 2~4 ns pulse-width and 30~50 mV amplitude at the pre-flashover stage. When approaching to flashover, the signals were turned into higher frequency one with pulse-widths of 1/2~1/3 ns and maximal amplitudes of about 70 mV, and these optical signals were immediately transformed into only one or two wave peaks having observably different amplitudes at the flashover stage. The optical signals reflected the development of streamer channels and changes of particles in channels in flashover. It is confirmed that photoionization is an important factor of flashover in transformer oil under nanosecond pulses.