The volt-time characteristic of the suspension insulator equipped in power system is an important property. It is used for the determination of protective level against the overvoltage. But in practical all suspension insulators are used in string insulator. Therefore, it is essential to obtain the characteristic of string insulator. In this study the insulators in string are porcelain insulator ANSI class 52-1 and 52-2, respectively. Under the standard lightning impulse voltage, positive and negative, the volt-time characteristics are obtained by statistical method. The results show that the increase of breakdown voltage is not proportional to the number of insulators in the string. For the positive impulse voltage, breakdown time is increased with the number of insulators. But it does not vary much for positive impulse voltage breakdown time, about 1.0-3.6 μs. Moreover, the mathematical model used to predict the volt-time characteristic of string insulator is proposed and it found to be well with the tested results. © 2014 IEEE.