Abstract: The streamer is the most critical and complex stage of the electrical breakdown, and is important in the determination of electrical breakdown characteristics of the electrode gap. The photographs, showed the path of the streamer propagation in air alone and along the smooth cylindrical insulator or insulator with shed, were taken by ultraviolet camera. The streamer propagated along smooth insulation surface with two propagation paths. That is ‘surface’ component propagating along the insulation surface and ‘air’ component propagating in air. As a shed appeared on the insulation surface, the ‘surface’ propagation path was cut off, hence, the ‘surface’ component of streamer never reached the cathode. The ‘air’ propagation path could bypass the barrier, and the ‘air’ component of streamer would cross the gap to reach the cathode. Through improving the applied voltage between the plates, the characteristics of flashover in air gap and along the insulation surface were investigated deeply. The relations between the streamer ‘stability’ propagation fields and the 50% breakdown voltage were analyzed. The flashover path along the insulators with a shed was shot by a camera, which was compared with streamer propagation path. Hence, the relation between streamer propagation path and the flashover path along the insulators with a shed was found.