In this paper the temporal evolution processes of large numbers of metallic balls under high voltage were observed using a PC-camera. The fractal dimensions of the stable agglomerates of metallic balls were calculated by means of the sandbox method. Also, the relationship between the fractal dimension and the voltage were studied. The result showed that the stable agglomerates of metallic balls can have dendritic fractal structures through a self-organization evolution process and the fractal dimension decreases with the increasing of voltage. The results are of value for the growth mechanism and morphology study of dissipative structure.