Abstract: The isothermal bainitic transformation in ductile iron welds and the effects of austempering temperature and time to the microstructure and mechanical properties of the weldswere systematically studied by means of X-ray diffraction, optical and electron microscopy.The experimental results show that the rate of bainitic transformation in ductil iron welds is fastand the weld structure mainly consisted of bainitic ferrite and retained austenite has high levelmechanical properties in the range of 15~240 min during austempering time. With increasing austempering temperature from 310℃ to 400℃, the amount of retained austenite increases andbainitic ferrite become wider. As the result, the plasticity of welds increases, but the strength of welds decreases.