Abstract: Based on the calculation formulae for load bearing capacity of concrete members strengthened with FRP (Fiber Reinforced Polymer) laminate, the effect of initial moment on strengthened beams is discussed under two different states- ultimate limit state (ULS) and limited stress state for concrete and steel reinforcement. Various factors for calculating load-capacity of strengthened beams are analyzed, respectively, such as concrete strength, reinforcement ratio, cross-section size, and FRP area ratio, in order to discuss the effect of various initial moments on the flexural capacity. The analyzed results indicate that the effect of initial moment on the load-capacity is less than 2 per cent, and the experimental results of large-scale concrete beams at different levels of sustained load shows that the effect of initial moment on the load-capacity is no more than 6.2 per cent. To ensure the safety of strengthening design, the designing formula under sustained load may consider a reduction coefficient (0.9) on the design moment of strengthened member without initial moment.