This　paper　presents　an　experimental　study　on　seismic　retrofit　of　circular　reinforced　concrete　(RC)　columns　with　FRP　composites.　Three　RC　columns　strengthened　with　Polythylene　Naphthalate　(i.e.,　PEN)　FRP　composite,　which　has　a　much　larger　rupture　strain　than　conventional　FRPs,　were　tested　under　combined　axial　loading　and　cyclic　lateral　loading.　Three　conventional　CFRP-confined　RC　columns　and　one　un-strengthened　control　column　were　also　tested　for　comparison.　The　control　column　was　found　to　fail　due　to　the　buckling　of　the　longitudinal　steel　reinforcement　and　be　of　shortage　of　ductility,　while　the　major　role　of　FRP　confinement　in　cases　of　slender　circular　RC　columns　is　to　prevent　the　spalling　of　concrete　cover　and　the　buckling　of　longitudinal　reinforcement.　Use　of　different　FRP　jackets　and　FRP　stiffness　had　a　marginal　effect　on　the　overall　load-deformation　responses　particularly　under　a　lower　axial　load　ratio.　Numerical　simulations　based　on　OpenSees,　into　which　a　cyclic　stress-strain　model　for　longitudinal　reinforcement　previously　developed　by　the　authors　was　implemented,　were　conducted　to　facilitate　an　in-depth　understanding　of　the　test　results　and　the　corresponding　strengthening　mechanisms.　The　cyclic　stress-strain　model　takes　into　account　the　buckling　effect　of　steel　reinforcement　and　the　lateral　confinement　effect　of　FRP　jackets.　Further　parametric　studies　revealed　that　under　a　high　axial　load　ratio　(e.g.,　0.4),　the　consideration　of　buckling　of　longitudinal　reinforcement　is　of　more　significance.