This paper introduces the use of a scanning electron microscope (SEM) to evaluate the insulation property of insulators by bombarding an insulator surface by an electron beam. A simple model is used to obtain the distribution of electric field on the bombarded surface and explain the occurring of a surface flashover in the form of a treeing. The duration of bombardment needed to initiate a tree information (hereinafter time to flashover treeing/TTF) was used to evaluate the insulation property. Under a certain SEMc energy and magnification, several MgO based insulators exhibited a different TTF. Therefore, this method can be used to evaluate the insulation property especially for insulators that are exposed in electron beam environment.