Compact Marx generators are widely used as portable pulse generator in different industrial applications, where high power pulses with rise times of order of some tens of nanoseconds are needed. For those applications, closing switches with very short delay and jitter times have to be used. Considering the fact that the breakdown of spark gap is a statistical process, the delay times of the closing switches are statistical voltage-dependent parameters. As a result, the characteristics of the output voltage of the Marx generator, i.e. its amplitude and rise time, are also statistical parameters. In this paper, taking the statistical behaviour of the breakdown process in spark gaps into consideration, the statistical characteristics of the output voltage have been studied in detail.