ABSTRACT. – To improve their freshwater algae removal performance, walnut and peanut shells were modified by citric acid. The modification effects of the concentration of citric acid, the heating temperature and the reaction time on walnut and peanut shells were studied using an orthogonal experimental design. The optimum modification parameters were obtained. The removal efficiencies of the modified walnut and peanut shells with regard to Microcystin aeruginosa cells were also investigated. The results showed that the modified walnut and peanut shells exhibited excellent algae removal performance. Compared with unmodified walnut and peanut shells, the efficiencies of algae removal were increased by 63.84 % and 63.34 % for modified walnut and peanut shells, respectively. Thus, the modification by citric acid can enhance the removal efficiencies of walnut and peanut shells with regard to M. aeruginosa cells, indicating that the modified shells have the potential to be used as algicides.