Bead-on-plate welding of pure copper sheet with a thickness of 200 μm were carried out with a 200 W blue diode laser. Output power was varied from 150 to 200 W. The spot diameter was set the 50,100 μm constant. The maximum laser intensity was 1×107 W/cm2. The threshold of the output power required for full penetration welding to 200 μm thick pure copper was obtained. Next, at each laser output power, the welding speed was changed to control the input energy, and the welding speed required for full penetration welding was determined. The maximum welding speed 35 mm/s was obtained with a laser intensity of 1×107 W/cm2. To investigate welding quality the bead appearance and crosssectional observation were performed.