Abstract: The pressure drop law study is made for different flow patterns based on the experiment on oil-water flows in horizontal pipeline for three highly viscous oils.On the basis of experimental data and theoretical analysis result, the experience correlations for effective viscosity are established and the pipe pressure drops with different flow patterns are calculated by means of pressure equations with effective viscosity and different velocity of flows.The study method used is suitahle for two-phase flow of heavy oil-water.The authors consider that the theoretical prediction model on effective viscosity should be established to make the study of two-phase flow of heavy oil-water more practical.