The　molecular　level　understanding　of　the　mechanism　about　the　3-methylthiophene　(3MT)　alkylation　with　isobutylene　(IB)　as　well　as　the　side　reaction　of　IB　dimerization　over　NKC-9　cation　exchange　resin　has　been　investigated　using　the　density　functional　theory　(DFT)　of　quantum　chemical　method.　A　model　of　benzene　sulfonic　acid　was　used　to　represent　the　cation-exchange　resin　catalyst.　Two　different　reaction　mechanism　types-stepwise　scheme　and　concerted　scheme　have　been　evaluated.　Activation　energies　of　each　reaction　path　which　were　obtained　from　the　DFT　results　have　been　improved　by　single-point　MP2　calculations.　In　the　stepwise　mechanism,　both　3MT　alkylation　and　IB　dimerization　proceed　by　adsorption　and　protonation　of　the　IB　to　form　a　sulfonic　ester　intermediate,　and　then　by　C-C　bond　formation　between　the　sulfonic　ester　intermediate　and　another　3MT　or　IB　to　give　the　reaction　products.　The　second　step　is　rate-determining　and　has　activation　barriers　of　148.41　kJ/mol　for　3MT　alkylation　and　160.52　kJ/mol　for　IB　dimerization.　In　the　concerted　mechanism,　the　reaction　occurs　in　one　step　of　simultaneous　protonation　and　C-C　bond　formation.　The　activation　barrier　is　calculated　to　be　169.10　kJ/mol　for　3MT　alkylation,　and　that　for　IB　dimerization　is　174.02　kJ/mol.　The　results　revealed　that　the　reaction　mechanism　of　3MT　allcylation　was　very　similar　to　that　of　IB　dimerization,　and　the　stepwise　mechanism　dominated　both　the　3MT　alkylation　and　IB　dimerization.　Moreover,　3MT　alkylation　is　more　easily　occurs　than　IB　dimerization　during　gasoline　alkylation　desulfurization.