The　circular-patch　welding　test　was　used　to　study　the　liquation　and　liquation　cracking　of　AZ-series　Mg　alloys.　A　heat　treatment　was　carried　out　on　the　as-received　AZ91　alloy　to　dissolve　the　gamma(Mg17Al12)　particles　before　welding.　The　circular-patch　welding　test　was　then　conducted　on　the　heat-treated,　as-received　AZ91　alloy　using　AZ61　and　AZ91　filler　wires.　The　results　showed　that　the　susceptibility　of　AZ91　alloy　to　liquation　and　liquation　cracking　was　significantly　reduced　by　the　dissolution　of　massive　gamma(Mg17Al12)　particles　via　a　heat　treatment　before　welding　as　the　liquation　mechanism　was　changed.　Both　constitutional　liquation　and　incipient　melting　occurred　in　the　partially　melted　zone　of　the　as-received　AZ91　welds,　while　only　incipient　melting　occurred　in　the　heat-treated　AZ91　welds.