The effect of Joule heat and mineral matter on conductivity of coal has been studied. The results show that temperature of coal-body increases because of the effect of Joule heat, and it makes electric current passing coal-body increases. With the increase of time, the effect of Joule heat weakens owing to emission of the heat and conductivity of coal decreases. The conductivity of demineralized coal is larger 22 or 86 times than that of original coal. The conductivity of coal is increased when its stack density is increased or graininess is decreased.