: Application of FRP composites is one of the most prevalent methods for retrofitting concrete structures. Strengthening of columns as a major member of structures which undergo concentric or eccentric loads is of considerable importance. The column under eccentric loading experiences lateral deflection at any particular column height which is defined as an eccentricity for the axial load at the both ends of the column. The lateral deflection would lead to secondary moment which its combination with primary moment will produce more significant effects. Although concrete columns are mostly under eccentric loading in practical use and are vulnerable to instability caused by slenderness effect, limited experimental studies have been conducted on the behavior of slender concrete columns strengthened with FRP composites in comparison with strengthened short columns. Previous studies state that the strength of columns increases with retrofitting with FRP, however, the strengthening effect of FRP on the column under axial-bending (P-M) loading would be reduced with increase in slenderness. In other words, the bearing capacity of FRP is not used completely and the column is vulnerable to instability and buckling. As a result of that, slenderness effect may refrain the column to achieve its maximum bearing capacity. This study is aimed to investigate the behavior of slender concrete columns retrofitted with FRP composites by the means of grooving method in order to lessen the slenderness effect observed in other retrofitting methods. For this purpose, eighteen circular concrete specimens of 130 mm in diameter and 500, 700 and 900 mm in height under eccentric loading with 60 mm eccentricity were tested. Half of the specimens are reinforced with normal strength steel () and half with high strength steel (HSS). All the specimens are retrofitted with FRP composites in longitudinal direction by the means of externally bonded reinforcement (EBR) and externally bonded reinforcement on grooves (EBROG) method. In EBROG method, 8 surrounding grooves with the size of 10 mm in depth and width and 460, 660 and 860 mm in length were cut and longitudinal sheets were installed. In this work, in addition to the employment of mechanical LVDTs for measuring displacements and strains, particle image velocimetry (PIV) method was used. The results obtained from these two methods are in great correspondence. Results showed that in a particular slenderness ratio, specimens retrofitted by EBROG method have more bearing capacity in comparison with EBR method. Moreover, slenderness influences the bearing capacity of retrofitted specimens with EBROG method more slightly compare to specimen retrofitted with EBR method or specimens of no retrofitted in the case of reinforced with and HSS strength reinforcement. Keywords: FRP composites, retrofitting, RC Column, Slender Column, Secondary Effect, Externally Bonded Reinforced (EBR), grooving method (GM)