To understand treeing phenomena in MgO/LDPE nanocomposite under ac voltage application, treeing breakdown properties and partial discharge inception voltage were investigated. Tree inception voltage increased with increasing filler content. Time to bridge counter electrode, time to breakdown and time-lag to breakdown also increased with addition of MgO nano-filler to LDPE. From these results, it becomes clear that the addition of MgO nano-filler gives positive effect for insulating material as far as the treeing breakdown characteristics and the suppression of the partial discharges are concerned.