Directional solidified (DS) turbine blades are widely used in advanced gas turbine engine.The size and orientation of columnar grains have great influence on the high temperature property and performance of the turbine blade.Numerical simulation of the directional solidification process is an effective way to investigate the grains growth and morphology, and hence to optimize the process.In this paper, a mathematical model was presented to study the directional solidified microstructures at different withdrawal rates.By using a Modified Cellular Automation (MCA) method and a simple linear interpolation method, the mushy zone and the microstrueture evolution were studied in detail.Experimental validations were carded out at different withdrawal rates.It is indicated that the withdrawal rate affects the temperature distribution and growth rate of the grain directly, which determines the final size and morphology of the columnar grain.A moderate withdrawal rate can lead to high quality DS turbine blades for industrial application.