Emulsification of crude oil and water may significantly affect the flow characteristics of the multiphase flow in gathering lines. Obviously, emulsification of a crude oil-water system is greatly affected by conditions such as shear intensity, water fraction, and temperature. However, there has been a lack of quantitative study of the emulsified water fraction of crude oil-water systems under various conditions of water fraction, temperature, and shear intensity. In this work, experiments were conducted by using a stirred vessel calibrated for the determination of mean shear rate, and the emulsified water fractions under flowing conditions were determined by extrapolation of the separated water volume to the moment when stirring was just terminated. Full emulsification of water was observed in a specific range of shear rates and below a critical water fraction, and only partial emulsification might occur above this critical water fraction. The range of shear rates in which full emulsification might occur became larger with a decrease in the water fraction, but it did not change with temperature in the studied range from 40 to 90 degrees C. For partial emulsification, the emulsified water fraction under flowing conditions was found to correlate well with the entropy production rate of viscous flow or energy dissipation rate regardless of the differences in water fraction, temperature, and shear rate, and a power law equation might well fit the data.