Abstract: With a long pulse low impedance electron-beam accelerator, the insulating properties of polytetrafluoroethylene (PTFE)， high molecular weight polyethylene (PE) and polycarbonate (PC) are measured in vacuum by the excitation of ～100 ns high-voltage pulses. The effect of grooved structure on enhancing the insulating strength is studied. Experimental results show that flashover delay time can be promisingly increased with proper grooved structure. PC is the most competent dielectric among the three tested materials, its flashover delay time can be increased by 90% with the grooved structure. The analysis by a scanning electron microscope (SEM) also shows that the surface properties of the materials, such as the roughness and homogeneity, are important in initiating a surface flashover.