Abstract: The high temperature brittle fracture caused by sulfur is one of key problems to restrict the production and the use of steel. Aiming at this problem, the effect of trace boron on the ductility and fracture mode of iron containing trace sulfur (mass fraction 0.006%) at high temperatures was investigated by tensile tests at high temperatures and SEM observations of fractured surfaces. The role of trace boron was discussed based on analysis of the segregation at interfaces by scanning Auger electron spectroscopy. The experimental results showed that the addition of trace boron increases markedly the high temperature ductility of iron containing trace sulfur and restrains the brittle fracture caused by the interface segregation of sulfur. Such effect of trace boron results from that trace boron can segregate to interfaces of iron, remarkably decreasing the segregation of sulfur and therefore restraining the formation of cavities at grain boundaries caused by intergranular segregation of sulfur.