Abstract: A planar spark gap switch with three electrodes which is suitable for connecting parallel-plate transmission lines is developed. The shape of positive and negative electrodes of the switch is semicircle, and the triggering electrode is a thin strip. When the planar spark gap switch substitutes for the stereo spark gap with hemispherical electrodes used in the exploding foil initiator, the parameters in the circuit are optimized obviously. The performance of the planar spark gap switch with different gap space was measured. The experimental results show that the inductance of the planar spark gap is less than 15 nH, and that the jitter time of the switch with different gap space is no more than 10 ns for different applied voltages ranges. When the planar spark gap switch is used in the puls