Abstract: Large-rupture-strain fiber reinforced polymers (LRS FRPs) have the large tensile rupture strain value of more than 5%. This characteristic might enhance the load-bearing capacity and ductility and provide a new choice for seismic strengthening of reinforced concrete (RC) columns. The FRPs provide lateral support for the longitudinal bars through the cover concrete, which might prevent or delay the buckling of longitudinal steel bars. However, the buckling of the longitudinal reinforcement might still be observed when the stirrup spacing is relatively large. Especially, for the FRP-confined rectangular columns, the ununiform confinement provided by the external FRP for the specimen might result in the buckling of longitudinal bars and reduce the load-bearing capacity of the specimen. In order to study the buckling behavior of longitudinal bars in rectangular RC columns confined by FRP, a total of 28 polyethylene terephthalate (PET) FRP-confined rectangular columns, comprising 16 RC columns and 12 plain concrete (PC) columns, were prepared and tested under monotonic axial compression. The effects of the thickness of external FRP jackets, stirrup spacing and section aspect ratio on the load-bearing capacity of FRP-confined RC columns were studied. To carry out a quantitative study on the buckling behavior of longitudinal steel bars in FRP-confined RC columns, the average axial stress-strain curve of a single longitudinal bar was obtained from the test results. The experimental results show that the application of PET FRP jackets can effectively improve the load-bearing capacity and ductility of RC columns. A large confinement stiffness and a low section aspect ratio close to 1 lead to a high confinement level for RC columns. And the lateral support of FRP can delay the buckling of the steel bars to a higher deformation level.