In this study, we studied to find the most suitable condition for the preparation of adsorbent by hydrothermal treatment of municipal solid waste incinerator (MSWI) fly ash. There are many variables to affect on the adsorption characteristics in our study. We discussed the effect with the variation of them, such as the concentration of NaOH, reaction temperature, reaction time, solid/liquid ratio and so on. The phase of adsorbent, its morphology and cation exchange capacity (CEC) were analyzed to evaluate their effects. The phase of adsorbent after treatment was identified by X-ray diffraction (XRD) and the morphology was examined by scanning electron microscopy (SEM), and CEC was analyzed by ammonium acetate method. The maximum CEC value was obtained under the condition of 3N NaOH, 100 degrees C of reaction temperature, 12 hr of reaction time, and 1:10 of solid/liquid ratio. The CEC was 68 meq/100g in this condition, which was 8 times higher than that of raw fly ash.