Abstract: By the tests on three reinforced concrete columns strengthened with external steel clad after exposure to ISO834 standard fire and three reinforced concrete columns without strengthening after exposure to ISO834 standard fire as well as three reinforced concrete columns without exposure to fire, the mechanical properties of reinforced concrete columns after exposure to ISO834 standard fire as well as the strengthening effectiveness of external steel clad are experimentally studied. Test results shows that after exposed to fire, the rigidity and load bearing capacity of specimens are all declined sharply compared with that at normal temperature without exposure to fire. In various loading stages from the initial loading to 80% ultimate strength, the ratio of axial rigidity of concentric columns after exposure to fire and at normal temperature was ranged from 0.22 to 0.37, the ratio of flexural rigidity of eccentric columns after exposure to fire and at normal temperature was ranged from 0.14 to 0.18, and the ratio of the load bearing capacity of specimens after exposure to fire and at normal temperature was ranged from 0.25 to 0.37. But the rigidity and load bearing capacity can be increased to a certain extent when strengthened with external steel clad. The ratio of axial rigidity of concentric columns strengthened with external steel clad and at normal temperature was ranged from 0.41 to 0.89, the ratio of flexural rigidity of eccentric columns strengthened with external steel clad and at normal temperature was ranged from 0.33 to 0.91, and the ratio of the load bearing capacity of specimens strengthened with external steel clad and at normal temperature was ranged from 0.97 to 1.06. The strengthening effect of external steel clad on load bearing capacity of reinforced concrete columns after exposure to fire is remarkable.