Abstract: The size effect on the seismic behavior of the reinforced concrete (RC) columns strengthened by carbon fiber reinforced polymer (CFRP) which subjected to low-cyclic loading was investigated. Three groups of geometrically similar RC columns with cross-sectional height ranging from 150 mm to 450 mm and a shear-span ratio of 3 were designed and tested. The influences of the number of CFRP layers, the size of the cross-section and the axial compression ratio on the seismic performance of the RC columns were also examined. It is indicated that: with the same structural size and axial compression ratio, the seismic behavior including the bearing capacity, the capacity of dissipated energy, the ductility capacity and the horizontal displacement of the CFRP strengthened RC columns are improved to different extents compared with the unreinforced columns, and the results indicate the existence of size effect; as the structural size of the tested columns increases, the dimensionless nominal bearing capacity of the CFRP strengthened RC columns is weaken and there is obvious size effect; as the structural size of the tested RC columns increases, the safety reserve coefficients decrease pronouncedly.