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.