Ab　initio　energetic　calculations　based　on　the　density　functional　theory　(DFT)　and　projector　augmented　wave　(PAW)　pseudo-potentials　method　were　performanced　to　determine　the　crystal　structural　parameters　and　phase　transition　data　of　the　polymorphic　rare-earth　sesquioxides　Ln2O3　(where　Ln=La-Lu,　Y,　and　Sc)　with　A-type　(hexagonal)　and　B-type　(monoclinic)　configurations　at　ground　state.　The　calculated　results　agree　well　with　the　limited　experimental　data　and　the　critically　assessed　results.　A　set　of　systematic　and　self-consistent　crystal　structural　parameters,　energies　and　pressures　of　the　phase　transition　were　established　for　the　whole　series　of　the　A-　and　B-type　rare-earth　sesquioxides　Ln2O3.　With　the　increase　of　the　atomic　number,　the　ionic　radii　of　rare-earth　elements　Ln　and　the　volumes　of　the　sesquioxides　Ln2O3　reflect　the　so-called　＇lanthanide　contraction＇.　With　the　increase　of　the　Ln3+-cation　radius,　the　bulk　modulus　of　Ln2O3　decreases　and　the　polymorphic　structures　show　a　degenerative　tendency.　©　2007　Elsevier　Inc.　All　rights　reserved.