This paper deals with a digest on electrical treeing degradation in nanocomposite of magnesium oxide （MgO） added to a low-density polyethylene （LDPE）. The objective is to elucidate the ＂visual＂ aspects of runaway stage of fi- nal treeing breakdown mechanisms, and to ensure that the existence of MgO filler itself is responsible for this aspect instead of voltage application changes. The ＂visual＂ aspect of treeing breakdown was investigated by applying ac ramp voltage with continuous rising speed of 0.5 kV/s. As soon as tree had been incepted, the voltage was kept constant to observe the tree propagation until breakdown. The tree propagations were also described by their fractal dimension. To ensure the responsibility of MgO filler on the ＂visual＂ aspect results, the experiment was repeated using the same rate of ac ramp voltage. However, as soon as tree had been incepted, the voltage was either increased or decreased to a constant value of mean tree inception voltage obtained from the preceding results to observe its propagation to breakdown. As a result, the tree inception voltage was increased with the filler concentration in LDPE. The filler is considered not only acted as a physical obstacle for tree to propagate, but also as a chemical obstacle in which the free electron might be trapped to cause difficulties for electron avalanche occurrence. This consideration could also be valid for tree propagation up to breakdown. During propagation, due to the filler, the tree structure is accompanied by more branches. Since the branches are closely placed each other, the tree tips become relatively uniform field compares to pure LDPE. This would produce the higher local breakdown strength at the tree tips, leads to the suppression of propagation. The ＂visual＂ aspect of breakdown could be achieved by applying voltage exactly at the inception level. After bridging the tree would not breakdown until a short period of time lag. Within this period, the tree channel will be eroded by internal flashover （IFO） and become thicker. MgO filler could still restrain the IFO at the small diameter; however there will be a maximum diameter above which the effect of MgO would be very small. Lastly, it is confirmed that the MgO filler itself excels to suppress the tree degradation instead of the voltage application changes. The polymer nanocomposite appears to be more resistive to treeing degradation than their base material.