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.