The effect of the chemical composition of nanocrystalline zeolites BEA on their physicochemical and catalytic properties in benzene alkylation with propylene is studied. It is shown that a decrease in the Al2O3 content in the reaction mixture during the synthesis of nanocrystalline zeolites leads to a decrease in the size of both the primary nanocrystals and their aggregates. The acidic properties of nanocrystalline zeolites BEA correlate with the aluminum concentration in the samples. The high concentration of acid sites of about 1400 mu mol/g and the developed surface of zeolites BEA represented by nanocrystal aggregates provide a high activity of the samples and a high selectivity for the target product-cumene-owing to a decrease in the contribution of side reactions, namely, the secondary alkylation and oligomerization of propylene.