The effect of the SiO2/A1203 ratio of H-mordenite on shape-selective catalysis was studied in the isopropylation of naphthalene with propylene. Aluminum concentrations at intracrystalline and external surfaces of H-mordenite are not directly related to catalyst performances. Dealuminated H-mordenite with a SiO2/A1203 ratio higher than 30 exhibited high catalytic activity and high selectivity for 2,6-diisopropylnaphthalene (2,6-DIPN). The enhancement of catalyst performances with the increase of SiO2/AI203 ratio is due to the suppression of coke deposition and the increase of shape-selective catalysis in the pores because the dealumination caused the decrease of acid density and strength. Coke deposition at the initial stage of the alkylation over H-mordenite with a low SiO2/A1203 ratio occurs at the pore entrances to inhibit the reaction in the pores. Low selectivity for 2,6-DIPN is due to non-selective catalysis at external acid sites which are active in spite of severe coke deposition. Naphthalene derivatives encapsulated in the pores showed that 2,6-DIPN was formed shape-selectively in the pores over all H-mordenites because of the minimum steric requirement at the transition state composed of substrates and acid sites, and that polyisopropylnaphthalenes in the pores were precursors of deposited coke.