Cobalt-doped Fe3O4 was prepared by a simple oxidation-precipitation method. Characterization results confirm that Co was successfully doped into Fe3O4. The doped Co increased the surface area, surface hydroxy groups and positive charge density of the product, which accelerated the decomposition of ozone into hydroxyl radicals. Cobalt-doped Fe3O4 exhibited high catalytic activity for the mineralization of 2,4-D by ozone in aqueous solution.