Abstract: In this paper, a theoretical study of the structures and proportics of 0.2% carlbon No 20 steel welded joints of flash welding and friction welding has been made. The paper provides two sets of concrete data of the properties of joints made by the two different welding tcchnological mcthcds. Taking into acccunt the dialcation thcory and the tendency of cold brittlcness of low carbcn steel, the authrs analyd the said differences of the properties and their cause of variation. Thrcugh TEM observation, highdensity dislocations and a small quantity of plate rrartcnsite were feund in the joints.