Abstract: The starting and finishing temperature of Acicular Ferrite (AF) in weld metal of microalloyed steels for different heat-input have been measured by using the thermal-expanding method. The curves of transformation have also been plotted. The temperature range of AF transformation is between 668℃ and 541℃. The T-t_c-f dynamics curves of AF transformation have been established based on the thermal simulating data and quantitative microstructure tests. The common rules of AF transformation in weld metal of microalloyed steels have been obtained by analyzing the dynamics curves.