Abstract: The thermodynamics driving force of aeicular ferrite(AF) transformation was calculated in theory using the model of KRC(Kaufman,Radcliffe,Cohen).The calculated result showed that the the oretical value of the starting temperature of AF transformation in weld metal was 1 100 K(827℃).And the actual value of the starting temperature of AF transformation in weld metal was 943 K(670℃).The thee retical value of the driving force of AF transformation in weld metal Was less than -570 J/m0l.The driving force of AF nucleation and growth increased with the decreasing of AF transformation temperature and the car-ben content in weld metal.The total driving force for AF trans formation was smaller than that of AF nucleation and growth.And the difference in-creased with the decreasing of AF transformation temperature.