This paper gives a brief introduction to the crystallization process and basic theory of computational flow dynamic (CFD), before discussing CFD simulation of batch crystallization of ammonium sulphate.Three types of impellers and two dimensions of tanks were applied in observing the changes of flow field.A popular CFD code, Fluent, was used to simulate stirred tanks used in the process, and the sliding mesh (SM) method was adopted to describe the rotating phenomenon.After comparison of the simulation results, an optimal crystallizer can be obtained.