Abstract: The penetration welding process was used to prepare T-lap joint by high power CO2 laser with 5083 aluminum alloy plates with 6 mm thick. The effects of laser power, welding rate, defocusing distance and the side gas flow on weld seam formability were studied. The results show that the penetration, weld width and joint width of weld seam increased with the increase of laser power and the decrease of welding rate, and the weld seam formability increased and then decreased. The defocusing distance and the side gas flow had less effect on size of weld seam, but the defects would appeared in weld seam if the choice for them was inappropriate.The fairly good aluminum T-lap joint could be obtained when the laser power was 10 kW, welding rate was 1 100 mm·min-1, defocusing distance was -2 mm and the side gas flow was 28 L·min-1, and the weld seam formability was good.