FRP composites have been widely used in retrofitting of reinforced concrete structures. Columns of a building play an important role in performance of the structure and its safety. Srengthening of columns by means of FRP composite and developing new methods in order to provide safe designs is therefore essential. Srengthening of RC columns is possible by means of FRP composite jackets around the column. The jackets are used mainly in a way that all or most of the fibers of composite are perpendicular to the column axis and in hoop direction to provide confinement. However, the axial compressive strength of columns can be increased by using FRP composite with longitudinal fibers; where the fiber sheets contribute by resisting compression directly. Buckling of FRP composite causes inappropriate performance of composite under compressive stresses. In longitudinally strengthened columns with FRP, buckling of composite causes debonding of FRP from concrete surface. On the other hand, researches have shown that the newly developed grooving method method as alternative to surface preparation can postpone or in some cases eliminate debonding of FRP sheet from the surface of the strengthened beams. Accordingly, in this study the grooving method was utilized for installing the FRP sheet with longitudinal fibers along reinforced concrete colum and its influence on postponing the buckling and debonding of FRP was investigated. In addition, the compressive stresses of fibers in composite at maximum load were determined and compared with its tensile strength. To perform the study, 29 reinforced concrete columns with 500 mm height were tested experimentally under uniaxial compression. 24 columns had circular sections of 150 mm diameter and 5 columns had 133*133 square section. Experimental results showed that using the grooving method by externally bonded reinforcement on grooves (EBROG) technique, increases the load carrying capacity of circular strengthened column with longitudinal composite by 14%. However, using conventional externally bonded reinforcing (EBR) method, did not increase the ultimate load capacity of the strengthened columns significantly. Furthermore, the compressive stresses of fibers in composite increased significantly by using the grooving method and it reached to 50 percent of tensile strength of fibers in flat coupon tests. Besides, considering the fact that tensile strength of fibers in real specimens is much lower than the strength of fibers in flat coupon tests, it was concluded that the fibers in composite achieve considerable compressive stresses. It was shown in this research that in circular columns strengthened with vertical fibers, using EBROG method, compressive stresses up to 80 percent of tensile strength of fibers in real FRP strengthened specimens is accessible. Key Words: Circular RC column, load carrying capacity, compressive FRP composite, buckling, grooving method, square column.