The grooving corrosion mechanism of an ordinary electric resistance welded (ERW) carbon steel pipe in sea water was investigated using electrochemical and microscopic observation techniques. Deep grooving corrosion was observed to occur at the narrow welded parts after rotating immersion test in sea water at 40°C for one year, and no grooving corrosion was observed in the matrix of the pipe. Potential measurement using micro-electrode showed that the potential of the weld was ca. 70mV lower than that of the matrix of the pipe. Optical and scanning electron microscopy revealed that the non-metallic inclusions, mostly manganese sulfide, were arranged connectedly at the outside and inside surfaces of the weld due to the concentrated and exposed metal flows. Observation of the grooving formation in 3% NaCl at 40°C using scanning electron microscope revealed that corrosion pits were developed in the initial stages around manganese sulfide inclusions stretched in the weld, and later the pits were enlarged by chemical dissolution of the manganese sulfide inclusions due to the lowering of pH in pits or by mechanically washing out of the inclusions. As these local corrosion pits were formed by most of the connected manganese sulfide inclusions in the weld, they grew up to the deep groove with the aid of macro-cell formed between the narrow weld and the matrix (wide cathode) of the pipe. On the other hand, in the matrix of the pipe corrosion pits were not developed around manganese sulfide inclusions. It is concluded that the manganese sulfide inclusions in the weld are accompanied by sulfur rich parts caused by rapid heating and cooling, and the sulfur rich parts are dissolved preferentially by the action of the local cell with manganese sulfide inclusions.