The vacuum arc root properties (temperature, current density, radius of an arc root, fraction of current density carried by electrons, evaporation rate etc.) of graphite and copper are calculated with the cathode-fall voltage as parameter and the arc root properties of graphite are compared with that of copper. Especially, there is big difference between the evaporation rate of graphite and that of copper. This reason is thought that the thermal conductivity of graphite is low and its evaporating temperature is high.