Abstract: An investigation into the structure-property relationship,cspeeially the embrittlement mechanism of the over-heated band of the simulated HAZ of a low carbon Si-Mn-Mo bainitic steel has been carried out with welding simulation,photomicroscope,TEM,SEM,EPM,X-ray diffraction,tension and impact and other tests.Results show that after various welding simulations,the grain size of austenite imposes the most significant effect on the tensile and impact properties;and the complex structure of small quantity of lower meta-bainite together with dominant low carbon martensite is of the best match of strength and toughness.With heat input increasing,the predominant factors affecting room temper ature toughness and low temperature toughness are the coarsened austenite grains and the segregation of carbon atoms at and near the grain boundaries at high temperatures,and the lalter leads to intergranular brittle fractures at high heat input.The M-A units in granular bainite and granular structure are not the cause of embrittlement.