Abstract: In this paper, it is illustrated that how to use the streaming potential method to determine the ξ-potential of Clay. A conductivity factor α,α=Kb/K+βKs,is introduced in order to overcome the difficulty which appears in application of the equationEs=PDξ/4πη（K+βKs） to soil. The physical meaning of the coefficient α and the determi-nation of a are demonstrated.The results of experiment on remolded Na-kaolin show that the effect of surface conductivity cannot be neglected if the concentration of electrolyte is lower than 10-1 N. The method given by this paper, then, possesses more value for most natural clayey soil.