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Laser bead on plate welding of 10 mm thick aluminium alloy under atmospheric and subatmospheric pressures were comparatively investigated. With the decrease of ambient pressure, the penetration depth increased sharply at first and then gradually levelled off. The largest penetration depth could reach 8?7 mm when welded under the pressures of 10 1 Pa, while only 4?9 mm under atmospheric pressure. Weld bead without any porosity was produced under ambient pressures of 10 21 Pa. The average tensile strength of joints welded under the pressure of 10 1 Pa was 300?2 MPa. The tensile strength remained constant as the ambient pressure decreased further. The shielding effect of plasma plume on laser beam was suppressed as the ambient pressure decreased. Therefore, the laser power deposition inside the keyhole was enhanced effectively. Under subatmospheric pressure, the porosity defects were eliminated effectively due to the keyhole stability and the change of liquid flow, i.e. moving upward along the rear wall of keyhole.