Abstract: The 3-D flow field with a central air core in a hydrocyclone is simulated by K-ε. turbulent model in the present paper. The model constants C1, C2, and Cμ are modified because ofthe anisotropic character of turbulent viscosity in the hydrocyclone. The numerical prediction has been tested against the LDA measurement results and has been found to perform well. Based on the prediction, the flow field and the pressure field as well as the distribution of the dissipation rate of the turbulent energy are discussed. The study shows that an important approach to reduce energy dissipation in the hydrocyclone is to improve the flow pattern in the cylindrical section.