In welding of steel, a part of base metal adjacent to weld metal is affected and hardened by welding heat. And the maximum hardness of this part of structural special steels is higher than that of low carbon steel. If the maximum hardness becomes higher, the cracking tendency in the affected zone increases. Accordingly, cracking in the affected zone easily occurs in structural special steels in comparison with the case of mild steel. It is an interesting problem whether the maximum hardness of the affected zone becomes higher or lower. when a pretty amount of arsenic is contained in the base steel. In this report, the authors investigated the effect of arsenlc content of the base metal on this maximum hardness and further on cracking in the affected zone of special steels, and from the experimental results they concluded as follows; a)The maximum hardness of the affected zone of low carbon steels increases with the increase of arsenic content. b)The maximum hardness of the affected zone of Cr-Mo steels or Si-Mn-Cr steels decreases with the increase of arsenic content. c)In case of the above mentioned structural special steels, the cracking tendency of the affected zone due to arc welding deposition decreases with the increase of arsenic content. And these facts can be explained sufficiently as follows. If arsenic exists in ferrite grains in the state of solid solution on the one hand the hardness of the ferrite grains increases, but on the other hand the quenching hardenability of such steel decreases when the steel is cooled from higher temperature A1 transformation point. It seems to be likely that the hardness of a low carbon steel is mainly influenced by the former factor but the hardness of the above mentioned structural special steel is influenced strongly by the latter factor. If the maximum hardness of the affected zone decreases with the increase of arsenic content of the base metal, the cracking tendency of the affected zone decreases.