Abstract: Nickel-iron type cast iron electrodes are widely used in welding QT60-2 nodular iron.Nevertheless,the research work has demonstrated that the strength of these welded joints is low.In this paper,the state of stress of welded joints under uniaxial tensile have been studid by the Moire fringe method.The total displacement,strain and stress of the welded joint were obtained.Based on the uneven distribution of the strain and stress,the mechanism of crack initiation and propagation was discussed.The fracture surfaces were examined by scanning electron microscope.A lot of spherical substances on the fracture surfaces were found,which may be another reason leading to cracking of the welded joint under low tensile stress.These new clues will help impreving the properties of nickel-iron type cast iron electrodes and the strength of this welded joint.