Aluminum casting is widely used in aeronautical, automobile and other industries nowadays. The Cellular Automaton (CA) method was modified to simulate the microstructure evolution of Al alloy casting. Simulated program code was developed and applied into Al casting production. A nucleation model was investigated based upon the experimental data. The solute diffusion in the liquid and solid phases was also considered in developing a grain growth model. With the developed models, not only grain structure but also dendritic microstructure can be predicted during the solidification process. The microstructure simulation of the Al alloy turbine wheel was studied in detail.