Recently, nanocomposite materials have attracted much attention in electrical treeing. In this study, we carried out a three-dimensional (3D) tree simulation using two macrosized fillers (i.e., spherical and discoid) as the first step in considering simulated trees in nanocomposite materials. Moreover, we carried out a comparison of simulated tree shapes depending on filler type. The results indicate that the simulated tree propagates more easily in composite insulating materials with spherical fillers than in those with discoid fillers from the viewpoint of the barrier effect.