This paper presents a study of electrical treeing phenomena in polymer nanocomposites. The polymer nanocomposite studied consists of epoxy resin as base polymer and silica as nano filler. Treeing experiments were performed at a constant ac voltage of 20kV, 50Hz on epoxy samples without any fillers as well as epoxy silica nano composites with 1% by weight of nano silica. Times for tree inception as well as tree growth patterns were studied. The results show that addition of small amount (1% by weight) of nano silica particles in epoxy resin can improve the treeing resistance by delaying the tree inception time as well as the time required by the tree to reach the opposite electrode. Treeing phenomena has been analyzed and interpreted by a physical model to explain the behavior in nanocomposites. The nature of bonding at the interface between epoxy and nano filler is characterized by using FTIR spectrometry. It has been shown that the type of bonding at the interface has an influence on the electrical tree growth pattern.