Abstract: The characteristics of grooved insulator flashover under pulsed voltage are discussed in this paper. The groove widths are determined by the secondary electron emission trajectory. The experiments results show that the flashover voltages increased while the insulator samples with 0.05 mm, 0.1 mm, or 0.7 mm wide grooves were used and the biggest voltage multiple was 1.4. The samples with 1 mm wide grooves had the same flashover voltage level as the flat ones. It is concluded that proper grooves design could increase the insulator flashover level by preventing the development of the secondary electron emission avalanche. Mechanism of inhibition was analyzed with calculation of electric field strength on the surface of insulator. Wider grooves, would trap the electrons and narrower ones would inhibit the motion of the initial electrons. Comparing the samples surface micrograms, a conclusion was given that the surface micro-flaw would drop the insulator flashover level.