Abstract: An experimental program was carried out to study the behavior of reinforced concrete columns strengthened by enlarging section area only on the single surface of the original columns after exposure to high temperature up to 800℃. A total of 11 specimens strengthened with different strength grade concretes were cast and tested with different loading positions. The tested ultimate load capacity, deformation and failure modes of specimens are presented in the paper. These test results can be used for accurately analyzing and designing reinforced concrete columns strengthened by using section enlargement method after exposure to fire.