Tree initiation behavior of an epoxy nanocomposite with 5 wt % nanoclay (layered silicate) was investigated in comparison to neat epoxy resin without fillers. To shorten the time for experiments, 600 Hz was used instead of 60 Hz, as acceleration for tree initiation had been confirmed at 10 kVrms and 14 kVrras between the two frequencies. V-t characteristics for tree initiation rather than tree growth to bridge the electrodes were obtained for conventional type of treeing specimens with an embedded steel needle subjected to voltages from 2 kVrms to 14 kVrms. As a result, the n value in V-n = t characteristics was confirmed to be 5.5 for neat epoxy and larger than 7 for nanocomposite. It was clarified that tree initiation V-t characteristics were improved by approximately one order of magnitude for the epoxy/nanoclay composite compared to the neat epoxy resin. To be precise, such an enhancement factor is one order at high field but even two orders at low field. Formed trees are field dependent. They are rather thick and short in shape at low field, but thin and long at high field. It is concluded from the analysis on the basis of interfacial models and other studies that initial trees are formed due to a PD erosion process at low field during a long time, but due to dielectric breakdown including charge trapping at high field for a short time.