The　fatigue　properties　and　life　prediction　of　2219　friction　stir　welded　joints　are　studied　based　on　the　microstructure-based　modeling.　The　joint　macro-micro　model　containing　the　crystal　characteristics　is　established.　The　stress　and　strain　responses　of　the　grains　are　obtained　through　the　crystalline　plastic　theory.　The　phenomena　of　stress　and　strain　concentration　easily　appear　at　the　grain　boundaries.　The　maximum　average　cumulative　shear　strain　is　in　the　weld　nugget　zone　(WNZ).　The　maximum　shear　stress　appears　in　the　heat　affected　zone.　The　lower　initial　yield　shear　stress　of　crystals　in　the　WNZ　makes　the　slip　system　activation　being　liable　to　happen　at　the　WNZ.　It　is　confirmed　that　the　weak　area　of　the　joint　is　the　WNZ　based　on　the　analysis　of　the　shear　strain　and　stress　distributions　and　the　initial　yield　shear　stresses　in　the　joint.　The　fatigue　life　is　predicted　based　on　the　established　joint　model.