The concentration distribution of droplets in liquid-liquid hydrocyclones was studied using a phase Doppler particle analyzer. The effect of cone angle and inlet flow flux on the concentration distribution of droplets was determined. The cone angle of the hydrocyclone obviously influenced the maximum droplet concentration, especially that in the cone section. The hydrocyclone with a cone angle of 6 degrees presented the best separation result. A higher inlet flow rate enhanced the separation of droplets from the water. The concentration profiles at five different axial heights presented a parabola law pattern in the radial direction. The optimized hydrocyclone was applied for the dewatering of vinyl chloride monomer (VCM). The amount of residual water could be greatly reduced.