One of the most common problems encountered in reinforced concrete structures is the non-conformity of the element properties and load capacity with the design expectations. Therefore, strengthening investigations that attempt to increase the load capacity of the defected structural element have become an important topic in the field of reinforced concrete studies, specially, for reinforced concrete columns. Recently, composite material such as fiber-reinforced polymer (FRP) started to become a good replacement for steel reinforcement for strengthening of reinforced concrete elements. In the eighties and nineties, the researchers and the construction industry showed great interest in this field and intensive researches were performed to begin the practical use of these materials. In this paper ten square columns of one third scale were tested and examined. These columns were loaded up to 0.3 or 0.5 of the nominal failure load. The load was kept constant at these values for about 24 hours. Columns were examined for the cracks and deformations, after that the columns were strengthened with glass fiber reinforced polymer (GFRP) or carbon fiber reinforced polymer (CFRP). Finally, the load increased till failure. The ultimate loads, mode of failure and energy absorption were recorded and analyzed.