Abstract: Model for microstructure of molten pool and HAZ was developed based on the dendritic growth kinetics and the grain growth theory using the cellular automaton method.The temperature field during TIG welding process was calculated by the finite element model. And the thermal cycle curves were applied to the CA model to calculate the dendrite growth in the molten pool and grain growth in the heat-affected zone. The simulation results showed that the nuclei at the fusion boundary mainly grows in the form of columnar dendrite toward the center of the weld. The final morphology depends on the partially melted grains of base metal at the fusion boundary and competition growing between dendrite arrays with different orientation. The microstructure at the center of the molten pool was equiaxed dendrites. The grain growth in the heat-affected zone results in the columnar structure of the molten pool coarser. The simulation results agree well with the experimental results.