Abstract: The experiments of vertical and flat position laser welding for 4 mm-thick 5A06 aluminum alloys were implemented, and the characteristics of weld dimension and porosity in the vertical position laserwelding for aluminum alloys were investigated.The results show that the concave value and excessive penetration value of vertical welding is less than those of flat welding.Further, with the increase of heat input, the difference of vertical and flat welding becomes obvious.The weld appearance and dimension of the vertical welding and flat welding were slightly different.When the heat input is increased to a great extent, the weld depth of vertical welding is more than that of flat welding.However, the weld width of vertical welding is less than that of flat welding.The porosity of vertical position laser welding for aluminum alloys is composed of the large and irregular porosity or hole.It is not obviously different during vertical welding and flat welding, and a great deal porosity concentrates in the upper and middle part of weld section, which can be indicatied from the distributing position and shape of porosity.The number of porosity in vertical welding was slightly less than that of flat welding for the same welding parametors.