Abstract: Dynamic observation of crack initiation and propagation in SG cast iron weld was carried out using a self-made uniaxial tensile device and optical microscope. Plastic deformation concentrates at the ferrite area under the tensile stresses. The areas where cracks initiate are as follows: (1) the ferrite areas with high strain concentration, especially the ferrite bridges among close graphite nodules and the thin ferrite areas with underface graphite nodules; (2) the ferrite-pearlite interface; (3) the pearlite areas with high strain concentration. The cracks tend to propagate in the area of high ferrite proportion through fracturing matrix bridges among graphite nodules and linking up the close nodules.