Abstract: The mathematical model was developed to account for the electro-viscosity function induced by electric double layer (EDL) in the light of EDL theory. The effect of the EDL function on the lubricating performance of water for a composite sliding block-disc pair was investigated in the presence of external electric field. The experimental results showed that EDL has apparent effect on the friction coefficient. The friction coefficient in the presence of the external electric field increased by about 20% at a relatively smaller entrainment velocity of 0.25 m/s, corresponding to a smaller film thickness. The friction coefficient predicted from the mathematical model considering the electro-viscosity function agreed well with the tested one. Thus, the established model could be used to well explain the effect of the electro-viscosity of the EDL on the lubricating performance of a liquid.