Abstract: Nd:YAG laser welding of 5A90 aluminium-lithium(Al-Li) alloy thin sheets has been carried out.Microhardness tests across the transverse section of the weld were performed.The influence of laser welding on the microstructure of 5A90 aluminium-lithium alloy at different sections has been evaluated by optical microscope,SEM and TEM.Microhardness tests revealed that the laser weld had lower microhardness than that of base metal.Laser welds revealed the formation and presence of a zone of equiaxed grains(EQZ) along the fusion boundary.The major strengthening phase in 5A90 aluminium-lithium alloy was δ'phase particles.The presence of the δ'precipitates in the welds was proved by the LI₂ superlattice reflections in diffraction patterns,but the volume of the δ'precipitates in the weld zone was very little,which attributed to the decrease in the microhardness of the weld compared with the base metal.