The properties of benzene adsorbed in all-silica Y and NaY zeolites have been studied by grand canonical Monte Carlo simulation. Biased particle insertions and deletions were implemented to allow the computation of equilibrium adsorption isotherms of benzene. Isotherms of adsorption at different temperatures were calculated and compared to the available experimental data. The calculated results show the amount adsorbed of benzene in NaY is much more than that in all-silica Y at the given range of pressure. The calculated heat of adsorption is in good agreement with experimental results. NaY preferentially adsorbs the benzene compared to all-silica Y. A distribution analysis of the adsorbed phase structure reveals the adsorption site in the zeolites.