Abstract: Microstructure,microhardness and fracture driving force are investigated experimentally and numerically for the heat affected zone of girth welds of X80 pipes in the paper.The soft region exists in the heat affected zone of girth welds,based on the experiment results of microstructure and microhardness distribution.By simplifing the girth welds with mechanical hetrogeneity into base metal,weld metal and heat affected zone,the fracture driving force of cracked girth welds are studied numerically with weld matching property,weld width and soft zone in heat affected zone.The results show that the fracture driving force decreases with the increase of weld matching property,and the soft region in the undermatched weldment has stronger effects on the fracture driving force of girth weld than that in the overmatched weldment.