The adsorption of 1-naphthol (NAP) in aqueous solution was investigated using unmodified and modified diatomite at different temperature, pH, adsorbent mass and contact time. The adsorption capacity of the modified diatomite significantly improved (about 2 times at 298 K) compared to the unmodified diatomite. The adsorption showed a good fit with the Langmuir model. The further data analysis demonstrated that the adsorption of 1-NAP on both materials could be best described by the pseudo second-order kinetic model. The thermodynamic parameters, including Gibbs' free energy (Delta G), enthalpy (Delta H) and entropy changes (Delta S) were determined. It was observed that the Delta H values (less than 40 kJ/mol) was positive, which indicated that the adsorption process is endothermic, physical adsorption. The Delta G values were negative, indicating that the adsorption of 1-NAP onto raw and modified adsorbent was spontaneous.