Abstract: Based on the fact that carbon-depleted regions come into being during the incubation period, thermodynamic models of diffusion and shear for weld acicular ferrite(AF)transformation in carbon-depleted regions of austenite were established respectively by using KRC (Kaufman, Radcliffe, Cohen)activity model and superelement algorithm, and used in the numerical simulation of a certain microalloy steel.The results show that with the variation of carbon content in the carbon-depleted region, the transformation mechanism of AF is different.The driving force in diffusion model is greater than that of shear model when the carbon content is much higher, while the driving force in shear model is a little greater than that of diffusion model when the carbon content is much lower.If there is no carbon in the carbon-depleted region, the driving forces of both models will be equal.The driving forces of the two models show the similar tendency, and increase with the decreasing of carbon content in carbon-depleted regions and AF transformation temperature.