Isopropylations of both naphthalene and 2-isopropylnaphthalene (S-IPN) were carried out over beta zeolite with various Si to Al ratios. Modification of the beta zeolite was also investigated. In the alkylation of naphthalene with isopropyl alcohol, the selectivity of diisopropylnaphthalene isomers (DIPN) was decreased from 60.5% to 25.8% by raising the SiO2/Al2O3 ratio from 25 to 160. However, the fraction of 2,6-diisopropylnaphthalene (2,6-DIPN) exhibited contrary trends. As high as 80% of the 2,6-DIPN fraction in total DIPN was obtained for a SiO2/Al2O3 ratio above 110. The results we’re correlated with the acidity and the particle size of the corresponding beta zeolite. Both ion exchange and chemical vapor deposition (CVD) modification methods reduced the activity of the beta zeolite. However, the selectivity of 2,6-DIPN was improved by CVD modification for isopropylation of both naphthalene and 2-IPN.