Cheletropic　addition　of　sulfur　dioxide　to　2,4-hexadiene　has　been　calculated　by　density　functional　theory　(DFr)　at　the　level　of　B3LYP/6-311+G**,　the　result　of　IRC　calculation　shows　that　the　cheletropic　addition　is　a　concerted　reaction.　In　reaction,　each　reactant　gives　its　electrons　in　HOMO　to　the　LUMO　of　other　reactant　simultaneously,　which　is　different　from　the　conditional　mechanism　of　4　+　2　cycloaddition.　Before　reaction,　the　difference　of　energy　level　between　HOMO　of　sulfur　dioxide　and　LUMO　of　2,4-hexadiene　is　largish(8.4　eV),　but　in　the　course　of　reaction,　the　antibonding　LUMO　of　2,4-hexadiene　turns　into　a　bonding　orbital　and　decreases　its　energy,　so　the　electrons　in　HOMO　of　sulfur　dioxide　can　flow　into　this　orbital　easily.　The　net　result　of　the　reaction　is　that　sulfur　dioxide　transfers　0.23e　negative　charge　to　2,4-hexadiene.