Abstract: The microstructure,subregion micro-hardness distribution and microwidth of flash welded HAZ of cold rolled low carbon sheets having different composition （steel A and B） were investigated. The results obtained show the microstructure characteristics of the welded HAZ of the cold rolled low carbon sheets （steel A and B） are same,but the ferrite grain sizes are different.On the basis of the microhardness variation,the HAZ is divided into five subregions. The microstructure characteristics change obviously from the fusion zone of the welded HAZ to the basis material with far away from the fusion zone. Compared with the steel B,the HAZ microwidth of the steel A is wider,but the microhardness is lower in contrast to the basis material. Therefore,the role of “restrain strengthening” reduces, the softening takes place in the HAZ. In contrast to steel A, the softening doesn’t take place in the HAZ of the steel B.