Investigative results and their practical application of the electric-arc microplasmatron for the thin-wall tubes welding are shown in the paper presented. Conditions for the qualitative seam welding are being discussed. The main attention is being paid to optimization of the discharge parameters and geometrical sizes of the tungsten cathode and the forming nozzle. The main reasons for arc shunting between the cathode and the nozzle were determined. To estimate the optimal values of the current, and to chose the diameters of the rod electrode and the nozzle, some original formulas are being suggested. Several practical recommendations are given to improve the plasma technology of the tubes welding.