Based　on　global　particle-swarm　optimization　algorithm　and　density　functional　theory　methods,　we　predicted　an　alloyed　Si2Ge　compond　with　body　centered　tetragonal　type　VII　clathrate　(space　group　I4/mmm)　built　by　a　truncated　octahedron　fromed　by　six　quadrangles　and　eight　hexagons　([4668]).　Si2Ge　clathrate　is　0.06　eV/atom　lower　than　VII　Si　clathrate　and　thermally　stable　up　to　1000　K.　It　has　an　indirect　band　gap　of　0.23　eV,　high　p-doping　Seebeck　coefficient　and　n-doping　electrical　conductivity.　It　owns　a　low　lattice　thermal　conductivity　of　0.28　W/mK　at　300　K　because　of　its　weak　bonding　and　strong　anharmonic　interaction　of　longitudinal　acoustic　and　low-lying　optical　phonons.　The　moderate　electronic　transport　properties　together　with　low　lattice　thermal　conductivity　results　in　a　high　optimal　thermoeletric　performance　value　of　2.54　(1.49)　at　800　(1000)　K　in　n　(p)-doped　Si2Ge.