Abstract: The molten metal in a horizontal position weld always tends to shed downward due to gravitation, which results in unevenness of the thickness in various layers of the weld and even undercut on the upper side and overlap on the lower side.Therefore to obtain good bead formation in such case is rather difficult. On the basis of heat-equilibrium of the molten pcol as well as the equilibrium of gravity force, an analysis was made on the weaving locus required to obtain a proper weld.It derives that the weaving frequency should vary in proportion with the welding current, the heat input would basically remain unchanged. A construction is thereby suggested, in which both the wire-feeding and weaving can be actuated by one motor. Finally, selection of optimum weaving locus parameters as well as welding conditions are introduced.