Directional solidified turbine blades of Ni-based superalloy are widely used as key parts of the gas turbine engines. The mechanical properties of the blade are greatly influenced by the final microstructure. In this paper, a mathematic model was proposed for the three dimensional simulation of microstructure evolution in directional solidification. Based on the thermo model of heat transfer, the grain growth within the blade and the microstructure morphology were simulated via a Cellular Automaton method. Validation experiments were carried out. The simulated cooling curves and microstructures corresponded well with the experimental results.