Welding　of　3mm-thick　5A90　aluminium-lithium　(Al-Li)　thin　sheets　was　performed　by　a　Nd:　YAG　laser.　Some　welded　joints　were　subjected　to　post-weld　heat　treatment　of　solution-aging　treatment　after　welding.　The　microstructure　and　hardness　distribution　of　the　as-welded　and　post-weld　heated　joints　were　investigated　comparatively.　Results　show　that　the　grain　morphology　of　the　weld　metal　is　randomly　oriented　equiaxed　dendrites　and　no　strengthening　precipitates　are　observed.　After　heat　treatment,　the　grain　morphology　varies　from　equiaxed　dendrites　in　as-welded　state　into　equiaxed　grains　after　the　post-weld　heat　treatment.　The　δ＇(Al3Li)　strengthening　precipitates　within　grains　and　a　smaller　interparticle　spacing　are　found　in　the　weld　metal　of　the　post-weld　heat　treatment.　The　hardness　in　the　as-welded　weld　metal　is　lower　than　that　of　the　base　metal.　The　post-weld　heat　treatment　gives　a　considerable　increase　in　the　hardness　due　to　the　precipitation　of　δ＇　phase　in　the　weld　metal.