Abstract: In order to investigate the effects of the adhesive layer and prestress on the flexural behavior of damaged steel beams strengthened with carbon fiber reinforced polymer (CFRP) plates, five damaged H-steel beams were tested under flexure. The characteristic load, load-deflection curve, CFRP plate strain and its strength utilization were analyzed. Test results show that the unbonded CFRP plate has a similar strengthening efficiency to the bonded CFRP plate, with a difference of less than 2% in the characteristic loads. The non-prestressed CFRP plate provides only a very small strengthening efficiency under the normal service state. However, the characteristic loads of steel beams strengthened with the prestressed CFRP plate can be significantly increased by about 30% compared with the non-prestressed CFRP plate. The plane section assumption is satisfied in the bonded CFRP plate-steel beam composite section while it is not satisfied in the unbonded CFRP plate-steel beam composite section. Compared with the non-prestressed CFRP plate, the strength utilization of the CFRP plate can be obviously increased when a prestress is applied in the CFRP plate. The developed finite element model can predict the flexural behavior of the specimens with good accuracy. The increase in the prestress, thickness and elastic modulus of the CFRP plate can increase the flexural strengthening efficiency of damaged steel beams.