Activated alumina (AA) prepared from alum sludge (AS) was used in removal of fluoride from aqueous solutions. The capacity of AA in the adsorption of fluoride from water was found to be 17.75 mg/g. The maximum efficiency of fluoride adsorption was 96% at the optimum condition (C = 5 mg/L, pH = 7, t = 20 min and adsorbent dosage = 5 g/L). Based on the results, it can be concluded that conversion of AS to AA can be a method for recycling of AS. Also AA can be used for adsorption of fluoride from aqueous solutions. So AS as a substance harmful to the environment turns into AA as a usable material in water treatment.