B3LYP　calculations　of　density　functional　theory　with　the　6-311　+　G(3df,2p)　basis　set　level　are　used　to　investigate　the　equilibrium　structures　and　intramolecular　rearrangement　reaction　between　the　linear　HSSH　and　branched　H2SS　isomers.　The　predicted　geometrical　parameters　and　scaled　harmonic　vibrational　frequencies　for　HSSH　are　in　good　agreement　with　the　available　values　experimentally.　The　predicted　S　-　S　and　S　-　H　bond　lengths　in　the　thiosulfoxide　structure　H2SS　are　0.　1986　and　0.　1366　mn　respectively　and　the　values　of　angle　SSH　and　angle　HSH　bond　angles　are　108.　3degrees　and　89.　5degrees　respectively.　The　transition-state　for　the　unimolecular　isomerization　is　suitably　characterized　by　diagonalizing　the　matrix　of　energy　second　derivatives　to　determine　the　unique　imaginary　vibrational　frequency　and　confirmed　by　the　IRC　calculation.　The　calculated　results　show　that　the　linear　structure　is　stable　with　respect　to　the　branched　form　(　lower　109.8　kJ/mol　corrected　with　zero　point　vibrational　energy)　energetically.　The　calculated　energy　barrier　for　the　direct　intramolecular　hydrogen　atom　transfer　isomerization　process　is　190.3　kJ/mol.　The　kinetic　results　demonstrate　that　the　isomerization　is　a　unimolecular　process,　but　the　reaction　rate　is　pretty　slow.　This　agrees　with　the　thermodynamical　results.　So　the　isomerization　process　should　proceed　via　the　other　likely　processes.