We measured the water content (0.01%-0.25%w/w) in crude oil emulsions using terahertz time-domain spectroscopy (THz-TDS). To improve the precision and range of the measurements, we used 1 and 10 mm thick quartz cells. The experiments were performed at 20 °C and the THz wave was transmitted vertically to the samples and detected on the other side. The experimental results suggest linear relation for the THz absorption coefficient and the water content of the crude oil emulsions in the observed range. The linear dependence facilitates high-precision measurements of the water content of crude oil. This suggests the potential of THz-TDS in determining the water concentration in crude oil and borehole fluid identification.