Ion migration characteristics of a rotary spark airgap of high voltage Pulse generator had been investigated. It was considered that the ion migration speed and the ions of the gases(atmosphere gases, $O_2,;N_2,,;and;H_{2}O,;etc$.) and the charged very fine particles(about $10~100nm$ size) migrated through the upper stator ball and bottom stator ball of the rotary spark airgap would determined the rise and fall times of the output high voltage pulse. In this paper, a basic study on the ion migration characteristics of the rotary spark airgap between the spark stator ball and the ion-sensing electrode of the proposed high voltage pulse generator have been investigated experimentally. As a result, the three kinds of ion speeds were detected by the ion-sensing electrode installed at the position of the bottom stator ball of the ball type sparkgap high voltage pulse generator. The migration velocities, diameters, masses, charges, numbers of the ions and particles were obtained by experiments and calculations, which, however, would determine the rise and fall times of the output high voltage pulse.