Abstract: The formation of "Ferrite Band" in the fusion zone of low aluminium alloy steel weld joints is a problem of general interest. The author has studied the mechanism of formation of the ferrite band, which usually occurs on the low Al alloy steel side near the fusion line after welding. The ferrite band is closely related to the difference in chemical composition of the pieces welded together, particularly to the difference in high temperature crystal structure which is governed by the chemical composition. The presence of carbon activity difference is the prerequisite for ferrite bard formation. High welding temperature and long time duration are the two principal terms of the dynamics of carbon diffusion. The ferrite band deteriorates the toughness, the high temperature strength, the resistance to stress corrosion, the resistance to electro-chemical corrosion and other properties of the weld joint. In order to eliminate or to minimize the ferrite band, we could, from a thermodynamlic point of view, resort to the adjustment of chemical composition, or to the minimization of the difference in the critical A3 points of plate material and weld seam. The mechanism of formation of the said band is in reality the same as that of the carbon migration in welds of dissimilar steels(the welding together of carbon steel, molybdenum steel and chrome-molybdenum steel, and also the welding of chrome-molybdenum steel to austenetic steel). They all follow the principle of diminishing carbon activity. The various chemical elements in different materials give rise to different degrees of carburization or decarburization of weld joints. The carbon migration in welding low Al alloy steel, which contains mainly non-carbide forming elements, is characterized by the formation of clearly visible ferrite band.