Electrical treeing is a pre-breakdown phenomenon responsible for the long-term degradation and failure of polymeric electrical insulation. This paper presents a study of electrical treeing in unfilled crosslinked polyethylene (XLPE), XLPE microcomposites (30, 40, and 50 wt% microsilica), and nanocomposites (1, 2, 3, 4, 5, and 10 wt% nanosilica) at an applied voltage of 12 kV. The XLPE nanocomposites show significant increase in electrical tree initiation and breakdown times compared to the microcomposites and unfilled XLPE. Notably, the 5 wt% nanocomposite shows the longest initiation and breakdown times and the least tree growth. The novelty of this study lies in the identification of nanosilica in the electrical tree channels via scanning electron microscopy. The role of micro and nanosilicas in electrical tree initiation and propagation is explained theoretically.