A combustion flame under the overhead power lines may be caused by breakdown disturbances in power systems. In this study, experiments were conducted for the purpose of investigating the reduction in insulation strength caused by combustion flame and the shape changes of the flame, and flashover characteristics and extinction phenomena of the flame in the simulated conditions of overhead power lines were examined under the application of a.c. and d.c. high-voltages. As the results of the experimental investigation, it is demonstrated that flame can remarkably reduce breakdown voltages of the air-gap in shorter range of the gap distance. As the gap distance increases, flame was quenched, before the flashover, by corona wind generated from the needle electrodes.