A size-dependent classification function, which is a parameter used to characterise crystallization in imperfectly mixed suspensions, was simulated using CFD. The CFX4.2 program was employed to simulate the local particle-size distribution in a multifluid flow using the k - epsilon turbulence model with respect to interface transfer between the particles and the solution. The problem was studied three dimensionally. A sliding grid technique was used to simulate the transient flow in the mixing tank. The size-dependent classification was calculated based on the local particle-size distribution. The factors that affect the classification function are discussed.