Abstract: The Computatinonal Fluid Dynamic(CFD) technique was used to describe the flow field in the 140 mm hydrocyclone, and the computational model was validated by Particle Image Velocity(PIV) measurement. Based on the results of simulation and experiments, the inner pressure, tangential velocity, axial velocity and radial velocity distribution within the hydrocyclone were characterized. Moreover, the transient flow field in the form of velocity vector maps of cylindrical part with different inlet velocities was obtained by PIV technology. The computational and experimental results show that the CFD simulation is deemed to be an adequate methodology for investigating the flow patterns inside the hydrocyclone and studying the separation mechanism; good qualitative agreements between computational and experimental results are achieved; the PIV measurement validates the accuracy of computational model.