Acid sites located on the external surface of H-ZSM-5 crystallites were selectively neutralized by a mechanochemical method with BaO supported on spherical silica particles. This effect was confirmed by both NH3-TPD measurement and comparing the catalytic properties of unmodified and modified H-ZSh4-5 catalysts in iso-propylbenzene and 1,3,5-tri-iso-propylbenzene conversion reactions. In methylation of methylnaphthalenes with methanol, the modified H-ZSM-5 zeolite catalysts exhibited lower initial methylation activities, higher selectivities to beta,beta-dimethylnaphthalenes, and longer catalyst lives than unmodified H-ZSM-5. This is ascribed to the decrease in methylation of alpha-methylnaphthalene and reduction in coke formation on acid sites on the external surface of H-ZSM-5 crystallites, especially in the initial period of reaction. Furthermore, by the aid of computer simulation, it is concluded that the shape-selective methylation of methylnaphthalene to beta,beta-dimethylnaphthalenes on H-ZSM-5 occurs inside the pore channels.