Abstract: A model describing the closing of an ultrafast gas switch was established. In the model the switch closing is divided into two phases: gas breakdown and voltage collapse. The development of the discharge in the first phase for hydrogen and nitrogen was numerically simulated. The waveform of the voltage applied to a 1mm nitrogen gap was assumed as a linear rising followed by a flat top. The time delay of the gap breakdown as well as the breakdown voltage was determined for different gas pressure and risetime of charging voltage. The results show a good agreement with that from experiments using hydrogen gap.