Upset welding of spheroidal graphite cast iron was carried out under various welding conditions. The diameter of the specimen was 10mm in diameter and 100 in length. Microstructure observation was employed in order to clarify the characterization at welding interface. Vicks hardness, tensile strength and elongation of joint were investigated. In the upset welding, the melted part on the joint was exhausted to the edge side, and the part becomes mottle structure. The microstructure on the joint becomes a spheroidal graphite cast iron structure in the pearlite matrix from the mottle structure with increasing heat input. When the melted part on the joint was exhausted to the edge side, the part where the spheroidal graphite coheres to the center part of the sample was observed
