The past decade has witnessed an ever increasing interest in strengthening, repairing, retrofitting, and upgrading of deteriorated concrete structures using fiber reinforced plastics (FRP). Enhanced load carrying capacity by FRP strengthening has been observed by a large number of researchers through experiments at ambient environments. In a harsh environment, however, FRP will degrade. This may result in structural degradation of FRP strengthened concrete members. The possible structural degradation has become a major obstacle for the wide-spread acceptance of this new strengthening technique. In the present study, boiling water and ultraviolet (UV) radiation are used to study the structural degradation of concrete beams strengthened with GRFP and CFRP fabrics. A total of eighteen 170 × 7.62 × 15.2 cm steel reinforced concrete (RC) beams are prepared as control specimens, conditioned specimens, and unconditioned specimens. The test results show that environmental attacks have a considerable effect on the structural degradation of FRP strengthened concrete beams. 57% ∼ 76% of the strengthening efficiency of FRP is lost after conditioning.