In this paper, modeling, procedure and algorithm using Monte Carlo (MC) technology were investigated respectively to simulate grain size and microstructure . First, two different kinetic model were defined by both experimental and statistics method. Then the procedure and algorithm were worked out based on MC technology. Thirdly, the grain growth process in HAZ was simulated, which has great influence on grain growth in HAZ. The result of the simulation demonstrates the grain growth process dynamically. Good agreement between MC simulation results and the experimental results was obtained which can provide a reliable evidence for evaluating the welding craft and the weldability.