Abstract:  Using the results from the studies of C-Mn steel ductile fracture mechanisms and the laws of the stress-driven migration of point defects in a stress field, and with an analysis of the micro-process of ductile fracture induced by the vacancy coalescence at high temperatures, a relationship between the fracture toughness J_\rm IC and temperature T in the intermediate temperature range is derived from the vacancy concentration. Experimental data of fracture toughness of pressure vessel steels are analyzed. It is shown that there is a good agreement between the theoretical model and the experimental results.