Abstract: The viscosity prediction of oil-water mixtures will be the key to predict oil-gas-water three-phase flow pressure drop if the correlation of gas-liquid two-phase flow pressure drop is being applied for the mentioned three phase flow models. An experimental investigation on the viscosity for oil-water mixtures under different water-cut in oil-air-water three-phase flow is carried out using local sampling method. The effects of flow pattern, air and liquid mixtures superficial velocity and water-cut on the viscosity of oil-water mixtures are also presented. By analyzing the experimental data, the variation law of oil-water mixtures viscosity is found in three-phase flow in horizontal pipes and stated as follows: the viscosity of wave flow is bigger than that of plug flow; the viscosity of plug flow is larger than that of bubble flow and stratified flow. It also shows that the viscosity of the mixture will increase when the reduced gas velocity and the reduced liquid velocity increases. Further, before the phase inversion the variation range of the mixture viscosity will increase as the water-cut increases. However, after the phase inversion the variation range of the mixture viscosity will decrease as the water-cut increases. If the section height is higher, the mixture viscosity will increase. In summary a new correlation for oil-water mixtures viscosity is proposed, and the numerical results are in relatively good agreement with the experimental data with an error of about -24.9%~0.93%.