This paper introduces the use of a scanning electron microscope (SEM) to evaluate the insulation property of insulators. An SEM may be used not only to observe a surface image but also to provide a fine electron beam for charging an uncoated-insulator surface at once. The distribution of electric field created by the surface charging can be developed by a simple model. The increase of electric field at the surface may exceed a critical value when a surface breakdown/flashover occurs. An insulation property is evaluated by varying the extended period of charging/electron bombardment which is needed to initiate a treeing-formation (hereinafter time to flashover treeing/ITF). In this paper, under a certain SEM's energy and magnification, Si02 addition into a high purity MgO has resulted in different TTF. Therefore, this method can be used to evaluate the insulation property of insulators which are exposed in electron beam environment.