Numerous studies showed that safety and serviceability of many concrete infrastructures and buildings built in 1970's have far less strength capacities than their design capacities and thereby require immediate retrofitting. Currently, these aged concrete structures are being repaired using many repair and strengthening methods developed in the past. Unfortunately, the previously developed repair and strengthening methods did not fully utilize the structural member's unique mechanical characteristics. Therefore, the researches on repair and strengthening methods developed based on composite mechanical behavior between repairing material and original substrate, which can effectively improve strength, ductility, and failure mechanism are needed at this time. In addition, these repairing and strengthening method must focus on structural member constructed using high performance concrete to meet the demand of present construction trend of building mammoth structures. Therefore, in this study, a repairing and strengthening method for retrofitting high strength concrete (HSC) columns that can effectively improve the performance of HSC columns is developed. The square HSC column's crosssectional shape is converted to octagonal shape by attaching precast members on the surface of the column. Then, the octagonal column surface is jacketed using Carbon Fiber Sheets (CFS). The method allows maximum usage of confinement effect from externally jacketed CFS, which result in improved strength and ductility of repaired HSC columns. The research results are discussed in detail.