Mg　alloys　have　been　extensively　used　in　lightweight　manufacturing　owing　to　excellent　mechanical　properties.　However,　liquation　cracking　occurred　during　welding　of　dissimilar　Mg　alloys,　significantly　deteriorating　mechanical　properties.　The　temperature-stress　distribution　and　generation　law　of　liquation　cracking　in　circularpatch　welding　of　dissimilar　Mg　alloys　were　investigated　by　a　combination　of　numerical　simulation　and　experimental　work.　The　peak　temperature　in　the　partially　melted　zone　0-2.5　mm　from　the　outside　of　the　weld　exceeded　710　K　(eutectic　temperature　of　AZ91),　which　is　prone　to　eutectic　liquation.　The　radial　tensile　stress　exerted　on　the　partially　melted　zone　during　the　cooling　process.　The　peak　value　of　radial　tensile　stress　in　the　last　stage　of　solidification　of　eutectic　liquid　phase　reached　about　24.4　MPa.　Heat　treatment　before　welding　was　used　to　dissolve　the　intermetallic　compounds　gamma(Mg17Al12)　to　prevent　liquation　cracking.