Decarburization of welded parts was usually observed when steels were butt-jointed in flash butt welding. A preventive method, that is, shielding the welding joint with argon gas proved unsuccessful so far, probably owing to the impurities of gas used and defects of the gas supplying apparatus. However, as a result of further experiments, transformer oil was used for avoiding the effect of oxygen in the air. By using this oil in a series of experiments the prevention of decarburization proved successful under specified conditions.Throughout the above experiments, high carbon steel rods of small diameter were used for the butt-joint with a Daiden flash butt welder.