To develop the water treatment process, the hydrocyclone is now used as a unit to operate. Understanding hydrodynamics is a key step to improve the separation process efficiency. Recently, a new simple method called the oil droplet method was proposed by Bamrungsri et al. [Chem. Eng. Res. Design 86, 1263-1270 (2008)] and applied to velocity measurements in a hydrocyclone. The Doppler Velocimetry measurements and computational fluid dynamics (CFD) have been proposed by many researchers as effective for studying the flow field of a hydrocyclone. This work presents a comparison of the experimental results from these two methods along with those obtained from numerical simulations. The numerical calculations of the 3D flow field were performed with FLUENT using the k-epsilon model and the Reynolds stress model (RSM). Measurements and CFD simulations were performed for two hydrocyclone configurations (5 and 10cm diameter). Doppler ultrasound velocimetry data and CFD-RSM results are in close agreement. The oil droplet method is less accurate for the continuous phase velocity profiles but is promising for the validation of Lagrangian tracking simulations.