© 2019 IEEE. Previous studies have evaluated the addition effect of micro-silica filler to the insulation performance of the epoxy resin. These studies state that micro-silica fillers can improve insulation performance by reducing the activity of electrical trees. However, these studies have not yet explained why electrical tree activity can decrease. To evaluate the causes of the decrease in this electrical tree activity, the PD activity and its charge behavior need to be evaluated. In order to carry out this evaluation, our research proposes an assessment method for PD activity and its charge behavior by observing individual partial discharge waveforms sequentially. There are two types of samples used: epoxy resin without filler and with micro-silica filler. Using PD measurement circuit made, the individual PD waveform parameters such as waveform shapes, PD types, number of PDs, and charge amount were collected and compared. For the result, it was found that the decrease in electrical tree activity in the silica filler sample can be caused by several reasons such as reduced number of PD occurring, decreasing effect of material degradation due to decreasing of local electrical field, and the division of energy in one PD event into smaller energy contained in several electron avalanches.