The mathematical model was developed to account for viscosity induced by surface tension in the light of EDL theory. The effect of the EDL on the lubrication performance of lubricant for a composite sliding block-disc pair was investigated in considering surface tension. The experimental results showed the EDL has apparent effect on the friction coefficient, The lower of the entrainment velocity, the greater of the friction coefficient. With the increase of entrainment velocity, the increase rate of friction coefficient decreases. Thus, the established model could be used to well explain the effect of the surface tension of the EDL on the lubricating performance of a liquid.