The formation of stable back beads in the first layer weld during one side multilayer welding is important to achieve high quality welded metal joints. The authors thus employed the switch back welding method for welding of V groove joints, from 2mm to 4.5mm root gap, without using backing plates. In this method, a power source, a wire feed unit and robot manipulators are the computer based cooperative control. In this robotic welding system, there are 4 personal computers, which control the welding robot, the digital welding power source and the wire feeder unit. Each unit is connected with Ethernet and UDP, User Datagram Protocol. By using this system, the authors investigate the relationship between the torch weaving and the pulsed welding current. For this purpose, the arc behaviors are taken with a high speed video camera. By analyzing these images, the waveform of the pulsed welding current is determined, i.e., in order to get a good back bead, it is important to discharge the arc to the root edge and the groove surface. The weaving frequency is up to 3Hz so that the switch back welding is applied to V groove backing less welding by using conventional welding robots.In the weaving frequency of 10Hz, 5Hz and 2.5Hz, the relationship between the pulsed welding current and the back bead shape are investigated. The suitability of the welding conditions for each root gap was verified by observation of the arc, molten pool and external appearance of back beads. A good quality of the welding was obtained under 2.5Hz weaving frequency. The feed-forward controller was designed to get a good back bead regardless of the gap variation. In order to verify the validity of the controller, the welding experiments were carried out. A wide and stable back bead was obtained regardless of the gap variation from 4.5 to 2mm.