GaAs Photoconductive Semiconductor Switch(PCSS) can be the main element in compact pulse power system, especially in Dielectric Wall Accelerator(DWA). The on-resistance of 3 mm gap GaAs PCSS is measured by using planar transmission line and coaxial-cable in order to study the effect of switch on-resistance on the switch performance. The results show that the measured on-resistance of GaAs switch is 14.9 Ω. The on-resistance of PCSS will cause the thermal damage to the switch, reduce the output capability of pulsed power system, and shorten the life time of GaAs PCSS as well.