Tree initiation time was evaluated by a pulse detection system developed by the authors to detect a small PD (partial discharge) signal that might take place at first just after a tree was formed. Six kinds of material were prepared for evaluation, i.e. base epoxy resin, microcomposite, two kinds of nanocomposites, and two kinds of nano-micro-mixed-composites. Lengths of trees detected were around 100 μm in our experiments. As a result, it was clarified that tree initiation time is prolonged by nano filler inclusion, but shortened by micro filler filling, and furthermore that nano fillers would help once-shortened time increase for micro filled epoxy specimens. PD erosion was evaluated for the same materials. It was confirmed that a good correlation exists between tree initiation time and PD resistance that is inverse to PD erosion. This correlation indicates that tree formation includes a PD erosion process. Detection of trees smaller than 100 um is required to clarify how a tree takes place at first.