Weld bead shape control is necessary for adequate welded structure performance when used under repetitive load. Bead shape occasionally deviates even when using automatic welding equipment because of changes in welding conditions such as the shape of the weld groove. A system for measuring the bead shape and controlling welding conditions during welding is thus required. Such a system will further upgrade the weld quality and welding efficiency. The effect of the root gap, a weld groove shape factor, on welding conditions and the bead shape were studied to work out a suitable bead shape control algorithm for an automatic bead shape control system to be used aluminium alloy butt welding.