Dependence of flashover voltage on gas pressure has been measured in vacuum and in atmospheric air using 10/30-nanosecond pulse power. The scope of gas pressure is between 8×10-4 Pa and standard atmospheric pressure. The laboratorial data show that the voltages are independent of the gas pressure under high vacuum（10-4～10-1 Pa）. As viewing from sorption and desorption

a conclusion has been made

that desorption begins early before the flashover and outgassing is the most important gas resource when gas pressure is in the range of high vacuum. Flashover is in fact a type of gaseous discharge process in a high-pressure phenomenon immediately above the insulator surface.