The digital inverter type pulse (DP) welding machine has currently used for MIG welding in these days. In comparison with conventional equipments, it is well known that the DP welding machine is superior in workability and weldability to conventional equipments. However, welders does not fully identify their optimum welding parameters. For example, the ratio of qualified persons divided by all applicants in the welding performance qualification(WPQ) slightly slight increase in this decade. Once they identify the optimum parameters of DP welding machine, it will be expected that the ratio will increase. Butt joints in flat position of the WPQ are nominated and optimum welding parameters are examined with the DP welding machine. Parameters of pulse waveform of current installed in the DP welding machine are used in this study. Thickness of aluminium alloy used is 3, 8 and 20mm respectively. And it was carried out the X-ray radiographic test and the bending test after completion of welding with optimum parameters. No imperfections are detected by radiographic test. The following are provided as examples of the optimum welding parameters. (1) Case of 3mm thick butt joint: welding current 100A(φ1.2mm wire), welding speed 6.7-10mm/s, 2layers-2passes. (2) Case of 8mm thick butt joint: welding current 200A(φ1.2mm wire), welding speed 3.3-5.0mm/s, 2layers-2passes. (3) Case of 20mm thick butt joint: welding current 200A(φ1.2mm wire), welding speed 3.2-4.8mm/s, 5layers-7passes. (4) Case of 20mm thick butt joint: welding current 250A(φ1.6mm wire), welding speed 3.3-5.0mm/s, 4layers-5passes.