Magnesium　alloys　has　a　wide　application　prospect　due　to　their　good　properties,　such　as　high　specific　strength　and　specific　stiffness,　but　the　susceptibility　of　liquation　cracking　is　also　pretty　high.　The　liquation　in　partially　melted　zone　of　AZ-series　magnesium　alloys　were　investigated　with　circular-patch　welding　test.　The　AZ91,　AZ31　base　alloys　were　welded　with　AZ61　and　AZ92　filler　wires　by　using　the　cold　metal　transter　metal　inert-gas　(CMT-MIG)　welding.　The　results　show　that,　the　liquation　occurred　along　the　weld　edge　of　AZ91　with　the　eutectic　reaction　occurring　between　gamma(Mg17Al12)　phase　and　Mg-rich　phase.　The　liquation　susceptibility　of　AZ31　was　pretty　low　as　gamma(Mg17Al12)　was　not　present　in　base　metal　of　AZ31.　Meanwhile,　a　new　method　for　predicting　liquation　cracking　based　on　binary　phase　diagram　was　proposed.　When　the　initial　solidification　temperature　of　weld　is　higher　and　the　solidification　temperature　range　of　weld　is　shorter　than　those　of　base　metal,　the　liquation　crack　susceptibility　of　weld　is　mostly　higher.　When　the　initial　solidification　temperature　of　weld　is　close　to　or　below　that　of　base　metal,　and　the　solidification　temperature　range　of　weld　is　close　to　or　longer　than　that　of　base　metal,　the　liquation　cracking　susceptibility　of　weld　is　lower.　This　method　worked　well　on　predicting　the　effect　of　composition　of　base　metal　and　filler　wires　on　liquation　cracking,　and　the　predicting　results　are　consistent　with　the　experimental　results.　That　is,　the　liquation　cracking　susceptibility　is　higher　with　AZ91　base　metal　used　than　that　with　AZ31　base　metal.　And,　the　liquation　cracking　susceptibility　is　lower　with　AZ92　filler　wire　than　that　with　AZ61　filler　wire.