Lithium-incorporated zeolite Ys (Li-NaYs) were directly synthesized under a hydrothermal condition from a gel mixture of Li2O . Na2O . SiO2 . Al2O3 . H2O. It was confirmed by XRD, NMR and elementary analysis, that lithium ions could be introduced onto the ion sites of zeolite Y during the course of crystallization. Usage of lithium citrate as the lithium source is the indispensable factor to achieve the synthesis. In adsorption of mixtures of 2,6-dimethylnaphthalene and its isomers, Li-NaYs exhibited greater adsorptive capacities and higher adsorption selectivities to 2,6-dimethylnaphthalene than NaY. Direct hydrothermal synthesis of Li-incorporated zeolite Y prevents the blockage of pore openings and/or channels during lithium ion-exchange. Furthermore, the difference in adsorptive properties between Li-NaY and Li ion-exchanged NaY is due to Li ions being introduced onto the S-1 ion sites, which can hardly be ion-exchanged, during the hydrothermal synthesis.