Refilled　friction　stir　spot　welding　(FSSW)　has　a　great　potential　to　be　a　versatile　process　for　mass　producing　high　strength　Al　alloys.　But,　the　liquation　cracking　susceptibility　of　refilled　FSSW　welds　is　high,　which　greatly　deteriorate　the　mechanical　properties.　In　this　paper,　the　microstructure　of　the　liquation　cracking　was　observed　and　the　cracking　mechanism　was　analyzed.　The　results　showed　that,　a　lot　of　hole-type-cracks　were　found　distributing　continuously,　as　a　chain,　along　the　grain　boundaries,　and　there　are　many　melted　intermetallic　compounds　around　the　holes.　With　EDS　analysis,　the　mechanism　of　liquation　cracking　during　refilled　FSSW　was　proposed:　when　the　eutectic　temperature　reached,　constitutional　liquation　was　initiated　by　the　eutectic　reaction　between　intermetallic　compounds　and　the　alpha-Al　matrix.　So　that　the　bonding　between　grains　was　weakened　and　the　cracking　along　the　grain　boundaries　could　be　caused　by　the　tension　induced　by　the　rotation　of　the　pin　and　sleeve.